Optical head and 00optical disk device

ABSTRACT

In the past there has been a problem that on a detection surface the influence of interference causes a defocusing signal to degrade, narrowing the range in which spherical aberration can be stably detected. Accordingly, a diffraction grating is used to focus the inner and outer sides of luminous flux on separate optical detectors before the optical flux is focused on an optical detector and defocusing signals are independently calculated to find the difference therebetween, thereby providing a spherical aberration signal. This makes it possible to detect spherical aberation signals more stably.

TECHNICAL FIELD

[0001] The present invention is related to an optical head and anoptical disk apparatus. More specifically, the present invention isdirected to a correcting technique as to thickness deviation of a baseplate, and also spherical aberration in two-sheet objective lenses for ahigh NA.

BACKGROUND ART

[0002] Very recently, while there are strong trends in high density asto optical disks, DVD-ROMs having storage capacities of 4.7 GB have beenmarketed with respect to CD-ROMs having storage capacities of 0.65 GB,corresponding to commercial-purpose reproducing-only optical disks.DVD-RAMs having storage capacities of 2.6 GB have already been availablein actual fields as recordable optical disks having large storagecapacities. Within a front half yearly period in HEISEI-era of year 12(A.D. 2000), DVD-RAMs having larger storage capacities of 4.7 GB will bepositively marketed. Such recordable DVDs, needs of various applicationsare made in such fields as, not only utilizations as storage mediadesigned for computers, but also storage media capable of recordingvideo images without rewinding/fast-feeding operations. At the end ofHEISEI-era of year 11, video recorders with employment of optical diskshave already been marketed. As to video records with employment ofDVD-RAMs, DVD-RAMs having storage capacities of 4.7 GB are expected tobe supported. Such video recorders equipped with DVD-RAMs are stronglyexpected in the market in view of compatibility between CDs andDVD-ROMs. However, the storage capacities of DVD-RAMs are not limitedonly to 4.7 GB, but may be desirably increased up to 20 GB, by whichhigh-definition moving pictures may be recorded on these DVD-RAMs for 2hours in connection with such a trend that satellite broadcastingprograms will be produced by using digital techniques.

[0003] Recording density of an optical disk is substantially limited bya dimension “λ/NA” of a recording/reproducing optical spot (symbol “λ”indicates wavelength of light and symbol “NA” represents numericalaperture of objective lens). As a consequence, in order to increase astorage capacity, a wavelength of light must be shortened, or anumerical aperture must be increased. As to wavelengths, very recently,development as to blue-violet-colored semiconductor lasers having awavelength of 410 nm has been progressed. Since the wavelength of thelaser used in presently-available DVDs having the storage capacities of4.7 GB is equal to 650 nm, if such blue-violet-colored semiconductorlasers are merely employed, then storage capacities of approximately 12GB may be in principle realized. The storage capacity of 12 GB isapproximately 2.5 times higher than the presently-available storagecapacity of 4.7 GB, namely a square of wavelength ratio. However, inorder to further increase the storage capacity of 12 GB to 20 GB, thenumerical aperture “NA” must be multiplied by 1.3, namely, the NA “0.6”of the presently-available DVD must be increased up to an NA of 0.78.

[0004] As the conventional techniques capable of increasing the NA, forexample, there is JP-A-11-195229 (first prior art). In this first priorart, the NA is increased up to 0.85 in maximum by employing thetwo-group/two-sheet of objective lenses. At this time, when the NA isincreased, there are such problems that the aberration is increasedwhich is caused by the shift in the optical system, and by the errorscontained in the thickness and the inclinations of the disk base plate.To the contrary, in the above-described prior art, in order to reducethe comatic aberration which is produced due to the disk inclination,the thickness of the base plate is made thin (up to 0.1 mm). Also, withrespect to the spherical aberration occurred due to the thickness errorof the base plate, the thickness of the base plate is detected from thedifference between the focal shift signal derived from the surface ofthe disk and the focal shift signal derived from the recording planethereof. Then, the interval between the two lenses is changed based uponthis detected thickness so as to compensate the spherical aberration.

[0005] Furthermore, there is JP-A-2000-057616 (second prior art) as another prior art. In this second prior art, as previously explained, thecontrol signal used to compensate the spherical aberration is detectedby the difference (subtraction) signal between the focal shift signalsbased upon the astigmatism method, which are detected by separating theinner side and the outer side of the optical spot on the photodetector.Also, at this time, the summation signal between these focal shiftsignals is used as the focal shift signal.

[0006] In the above-explained first prior art, the spherical aberrationis detected in such a manner that the thickness of the base plate isdetected from the focal shift signal derived from the surface of thedisk and the focal shift signal derived from the recording film planethereof. However, in this case, since the spherical aberration is notdirectly detected, there are other problems that the errors readilyoccur due to adverse influences caused by the deviation of refractiveindexes of the base plate and the shift of the photodetector, and thecontrol operation can be hardly carried out.

[0007] As will be explained later in detail, in the second prior art,there are such problems that the waveforms of the focal shift signalsare largely deteriorated which are caused by the spherical aberrationitself, and the focal shift range capable of detecting the sphericalaberration is narrow. Furthermore, the offset of the focal shift signalcaused by the spherical aberration is also large.

[0008] The present invention has been made to solve the above-describedproblems, and therefore, has an object to provide an optical diskapparatus capable of detecting spherical aberration in higher precisionand under stable condition, which is caused by deviation of a base platethickness and a shift in an optical system, and capable of correctingthis spherical aberration, and also capable of detecting a focal shiftsignal having a small offset so as to record/reproduce an optical diskunder stable condition.

DISCLOSURE OF THE INVENTION

[0009] (Solving Means)

[0010] An optical head of the present invention for solving theabove-explained problems, is basically arranged by a semiconductorlaser; an optical system for condensing laser light of the semiconductorlaser onto an optical disk; a variable focal point mechanism for varyinga focus position of the condensed light; a spherical aberration addingmechanism for adding variable spherical aberration to the condensedlight; an optical branching element for branching reflection lightreflected from the optical disk form an optical path defined from thesemiconductor laser up to the optical disk; a lens for condensing thebranched reflection light; and a light receiving element for receivingthe light condensed by the lens so as to convert the received light intoan electric signal.

[0011] At this time, a second branching element is additionallyprovided, while the second branching element branches reflection lightbranched by the optical branching element in such a manner that thesecond branching element further separates this branched light intofirst luminous flux located in the vicinity of an optical axis andsecond luminous flux located at a peripheral portion of the opticalaxis, and both the first luminous flux and the second luminous flux arecondensed to the light receiving element. The optical branching elementessentially constitutes a hologram. As the spherical aberration addingmechanism, an electrostatic actuator capable of varying an interval oftwo-group/two-sheet of objective lenses, or a liquid crystal filtercapable of electrically controlling a phase of transmission light isemployed. Also, a summation signal between these two focal shift signalsis assumed as a focal shift signal. The variable focal point applyingmechanism is controlled by employing this focal shift signal. As thevariable focal point, an electrostatic actuator which essentially mountsand moves an objective lens is employed.

[0012] At this time, while the first and second optical branchingelements are constructed in an integral form, the optical system may besimplified.

[0013] Also, since the optical branching element formed in the integralform is constructed of a polarizing hologram, a loss in a light amountmay be suppressed.

[0014] Also, while the first optical branching element is constituted bya polarizing element and the spherical aberration applying mechanism isconstituted by a liquid crystal element, the liquid crystal element isarranged between the semiconductor laser and the first optical branchingelement, so that the optical head may be made compact. Since the liquidcrystal element is employed only in the going optical system, a loss ofa light amount can be suppressed. In this case, a polarizing elementimplies an optical element such as a polarization beam splitter and apolarizing diffraction grating, which owns an incident polarizationdepending characteristic with respect to a light amount ratio to bebranched.

[0015] Also, while the first optical branching element is anon-polarizing optical branching element and the spherical aberrationapplying mechanism is a liquid crystal element, this liquid crystal isarranged between the first optical branching element and the objectivelens. As a result, the optical head can be made compact. Also, since thespherical aberration caused by the liquid crystal is effected in thereciprocative optical path, an adverse influence such as an offset (willbe discussed later) of the spherical aberration can be avoided. In thiscase, a non-polarizing element implies an optical element such as anon-polarization beam splitter and a non-polarization diffractivegrating, which has no incident polarization depending characteristicwith respect to a light amount ratio to be branched.

[0016] Also, as to an optical element between the first opticalbranching element and the photodetector, which gives no adverseinfluence to the optical system defined from the semiconductor laser tothe objective lens, such an optical element as a lens which causes thespherical aberration is not arranged. As a result, the offset neveroccurs in both the spherical aberration to be detected, and thespherical aberration on the optical disk plane.

[0017] Also, since both the objective lens and the spherical aberrationapplying mechanism are fixed in an integral form, it is possible toeliminate an adverse influence of an axial shift of spherical aberrationwhich is caused by lens decentering in connection with the trackingcontrol.

[0018] Also, while an effective luminous diameter of the objective lensis smaller than, or equal to 1 mm, the semiconductor laser, thespherical aberration applying mechanism, the first and second opticalbranching elements, the objective lens, and the photodetector are fixedin an integral form to be mounted on the variable focal point mechanism.As a result, the optical head can be made compact, and further, it ispossible to eliminate the adverse influence of the axial shift of thespherical aberration correction which is caused by the lens decenteringin connection with the tracking control. If a thickness of a base plateof an optical disk is 0.1 mm, even when the effective diameter of theobjective lens is 1 mm, then the working distance longer than, or equalto 0.1 mm can be securred by employing one sheet of lens. The ground whythe effective luminous flux diameter is smaller than, or equal to 1 mmwill be explained with reference to FIG. 33. FIG. 33 indicates acalculation result of the working distance with respective effectivediameter under such a condition that an NA is 0.85; a thickness of adisk base plate is 0.1 mm; a refractive index of the base plate is 1.62;a refractive index of the objective lens is 1.8; and also, a radiuscurvature of a first plane of a single type objective lens is equal to ahalf of the luminous flux diameter. In this case, based upon Japanesebook “Lens Designing Method” (written by MATUSI, published by KYORITSUpublisher, No. 7, 1989), the working distance “WD” was calculated undersuch a lens thickness that a value of spherical aberration becomesminimum, which is conducted from the aberration theory in the analyticmanner. Such a condition that the first plane radius curvature is equalto ½ of the luminous flux diameter corresponds to a sever conditionunder which a lens can be geometrically established. However, in anactual case, since the lens is a non-spherical shape, if a distance isapproximated to this non-spherical shape, then the lens can beestablished. As a result, even when NA is equal to 0.85 and thethickness of the base plate is equal to 0.1 mm, it can be seen that sucha condition may be secured. That is, the effective diameter is 1 mm, andthe working distance is approximately 0.1 mm. This condition is nearlyequal to the working distance of 0.13 mm as to the two-sheet of lenseshaving the effective diameter of 3 mm shown in FIG. 11, namely can besufficiently realized.

[0019] Also, in such a case that the spherical aberration applyingmechanism and the object lens are not formed in an integral manner, whenthe objective lens is moved along the radial direction for the trackingcontrol operation, the axis of the spherical aberration which isproduced by the spherical aberration applying mechanism is shifted fromthe optical axis of the objective lens, so that comatic aberration mayeffectively occur. Since a comatic aberration applying mechanism isadded, this comatic aberration can be compensated. Also, in such a casethat the spherical aberration applying mechanism and the objective lensare formed in the integral manner, this comatic aberration applyingmechanism may become effective with respect to such a comatic aberrationoccurred in the case that the base plate of the optical disk isinclined.

[0020] Also, in the case that the optical head is made compact, sincethe semiconductor laser chip is constituted on the substrate of thephotodetector in an integral manner, the optical head can be easilyassembled and adjusted.

[0021] Also, in such a case that a track pitch of an optical disk isnarrow, since the spherical aberration is detected/compensated incombination with the tracking operation by the differential push-pullsystem, the offset caused by the movement of the objective lens inconnection with the tracking control operation can be canceled. In thiscase, such a diffraction grating is provided in luminous flux directedto the objective lens, and this diffraction grating may diffract both aninner-sided luminous flux and an outer-sided luminous flux alongdifferent directions. Also, the diffraction grating diffracts theouter-sided luminous flux along a substantially tangential direction ofthe optical disk, and also diffracts the inner-sided luminous flux alonga substantially radial direction. In particular, the outer-sidedluminous flux is arranged on both sides of zero-order light which is notdiffracted, and is shifted may be an essentially ½ period of the guidegrooves, or the pit strings of the optical disk. In this case, theexpression “essentially” may give the following implication. That is,shifts between the zero-order light and the ± first-order light isessentially identical to each other even in the ½ period, and also evenin such a period of (n+½), in which symbol “n” indicates an integer.Also, even when the azimuth of the diffraction grating is adjusted byconsidering the ½ period, effects are essentially identical to eachother with such an error range (on the order of ±⅛ period) where noadverse influence is given to the signals. Similarly, the expression“essentially tangential direction” implies such a tangential directioninvolving the above-described “essential” shifts.

[0022] Since the above-described operation is carried out, such atracking signal is obtained from the outer-sided luminous flux, whilethe tracking signal has the same polarity as that of the zero-orderlight as to the offset by the lens movement, and the tracking signal hasan inverted polarity as that of the zero-order light. As a consequence,since a differential output is produced from these tracking signals,such a tracking signal from which the offset is canceled may beobtained. In the case that the track pitch is made narrow, since theadverse influence of the interference caused by the diffraction light isnot present in the inner-sided luminous flux, the inner-sided luminousflux is arranged along an essentially radial direction, the diffractionangle of the diffraction light can be easily suppressed within theallowable range as to the view angle of the objective lens. In thiscase, the expression “essentially radial direction” owns the followingimplication. That is, as previously explained, since the adverseinfluence of the interference caused by the diffraction light is notpresent in the inner-sided luminous flux, there is no relationship as tothe relative position between the guide grooves and the inner-sidedluminous flux. Therefore, when the inner-sided luminous flux isdetected, this inner-sided luminous flux may be separated from theouter-sided luminous flux.

[0023] Also, in the case that the separation between the inner-sidedluminous flux and the outer-sided luminous flux is carried out in thereflection optical path from the optical disk, since the astigmatismmethod is employed as the focal point detecting system, a total numberof signal output lines derived from the photodetector can be reduced. Insuch a case, in the diffraction grating used to separate the inner-sidedluminous flux from the outer-sided luminous flux, such a pattern forapplying astigmatism may be employed.

[0024] Also, an optical disk apparatus for solving the above-describedproblem is arranged by at least the above-described optical head, and acalculation circuit for acquiring a reproduction signal and a focalshift signal from an electric signal of a light receiving elementthereof. Then, the first and second focal shift signals areindependently detected as to the above-described first luminous flux andthe second luminous flux, and then, a signal which is substantiallydirectly proportional to the spherical aberration is obtained from asubtraction signal obtaining by essentially subtracting these first andsecond focal shift signals. This signal is used to control the sphericalaberration applying mechanism so as to reduce the spherical aberrationof the condensed spot. In this case, the expression “essentially”related to the calculation of the spherical aberration detecting signalowns the following implications. That is, the sequence of the circuitcalculation may involve not only such a calculation sequence that afterthe focal shift signal of the first luminous flux and the focal shiftsignal of the second luminous flux have been firstly and independentlycalculated, the difference signal is calculated between these focalshift signals, but also another calculation sequence in such a way thata calculation result may become essentially equivalent to theabove-described calculation result (for example, all of components whichcontribute polarity of “+” with respect to result, and all of componentswhich contribute polarity of “−” are added to each other, andthereafter, a difference between these added components is calculated).

[0025] Also, in such an optical disk apparatus, when this optical diskapparatus is employed in an optical system where a liquid crystalelement, or the like is combined with a polarization beam splitter and a¼-λ plate, such a phase difference is applied by the liquid crystalelement only to a linearly polarized light component along onedirection, spherical aberration may be effected only to luminous flux ofa going optical path, which is directed to the optical disk. In thiscase, this ¼-λ plate implies such an optical element having two opticalaxes located perpendicular to each other, through which linearlypolarized light entered thereinto directly passes with maintaining thislinearly polarized light. This optical element may apply a phasedifference of a ¼-wavelength with respect to incident light of two setsof linearly polarized light which are located perpendicular to eachother and are polarized along directions of the respective optical axes.Such an optical element may have an effect capable of converting thepassing light into the circularly polarized light in the case thatlinearly polarized light located in parallel to two optical axes andhaving the same amplitudes and the same phases are entered into thisoptical element, namely in the case that linearly polarized light isentered which is inclined by 45 degrees with respect to the opticalaxis. This is because when the light directed to the disk passes throughthe ¼-λ plate, this light becomes circularly polarized light, and whenthe luminous flux which is reflected from the disk to be returned againpasses through the ¼-λ plate, this reflection light becomes such alinearly polarized light along the polarization located perpendicular tothe going optical path. However, as to the spherical aberration which isdetected by the photodetector and is obtained by way of the calculation,the spherical aberration applied by both the going optical path and thereturning optical path is reflected. As a result, when this signal isessentially and directly fed back to the spherical aberrationcompensating mechanism, the spherical aberration of the reciprocativeoptical paths is compensated only in the going optical path, and then,it is so controlled in such a manner that there is no sphericalaberration on the photodetector. Accordingly, only the sphericalaberration of ones optical path is originally applied to the spot on theoptical disk, whereas the spherical aberration in the reciprocativeoptical paths is compensated only in the going optical path. As aconsequence, the spherical aberration is left in the inverse code, theamount of which is equal to the original amount of the sphericalaberration. Therefore, in order to avoid this difficulty, the feedbacksystem is arranged in such a manner that the spherical aberrationoccurred on the disk plane becomes zero. In this case, the expression“essentially” implies, as previously explained, such another calculationsequence that the sequence of the circuit calculation is performed toobtain essentially equivalent calculation result.

[0026] To this end, for example, such a loop is provided by which adrive signal for electrically driving the spherical aberration applyingmechanism is fed back to a system for amplifying the sphericalaberration error.

[0027] Furthermore, in these optical disk apparatus, since only thespherical aberration on the disk plane is compensated, the sphericalaberration of the returning optical path is not compensated, so that theoffset is produced in the focal shift signal in this case. In order tocompensate the offset, and to compensate the offset of the focal shiftsignal in response to the detected spherical-aberration signal, thespherical aberration error is multiplied by a proper coefficient, andthe multiplied spherical aberration error is added to the drive signalof the variable focal point mechanism so as to drive the variable focalpoint mechanism.

[0028] As previously explained, in such a case that the sphericalaberration applying mechanism and the object lens are not formed in anintegral manner, when the objective lens is moved along the radialdirection for the tracking control operation, the axis of the sphericalaberration which is produced by the spherical aberration applyingmechanism is shifted from the optical axis of the objective lens, sothat comatic aberration may effectively occur. In the optical diskapparatus using the optical head equipped with the comatic aberrationapplying mechanism, the move amount of the objective lens is detected,and the comatic aberration applying mechanism is driven by using thisdetected move amount so as to compensate this comatic aberration.

[0029] Also, for instance, in order to detect the move amount of theobjective lens, an unbalance of the branched luminous flux is detectedwhich is located in the vicinity of the optical axis along the radialdirection of the optical disk. In such a case that a track pitch of anoptical disk is sufficiently narrow, diffraction light caused by guidegrooves of the disk is deviated at the peripheral portion of theluminous flux, the adverse influence of the interference caused by thesediffraction light is not present in the luminous flux in the vicinity ofthe optical axis. As a consequence, the light in this region isindependently detected within two regions which are subdivided by thediameter along the tangential direction of the disk, and then, adifference between these detected light is calculated, so that themovement of the objective lens along the radial direction in connectionwith the tracking operation can be detected.

[0030] Also, in the case that a track pitch of an optical disk is madenarrow (namely, when track pitch is smaller than, equal to λ/NA μm withrespect to numerical number NA of objective lens and wavelength λ),since the spherical aberration is detected/compensated in combinationwith the tracking operation by the differential push-pull system, theoffset caused by the movement of the objective lens in connection withthe tracking control operation can be canceled. As previously described,in this case, such a diffraction grating is provided in luminous fluxdirected to the objective lens, and this diffraction grating maydiffract both an inner-sided luminous flux and an outer-sided luminousflux along different directions. Also, the diffraction grating diffractsthe outer-sided luminous flux along a substantially tangential directionof the optical disk, and also diffracts the inner-sided luminous fluxalong a substantially radial direction. In particular, the outer-sidedluminous flux is arranged on both sides of zero-order light which is notdiffracted, and is shifted only by the ½ period of the guide grooves, orthe pit strings of the optical disk. Since the above-described operationis carried out, such a tracking signal is obtained from the outer-sidedluminous flux, while the tracking signal has the same polarity as thatof the zero-order light as to the offset by the lens movement, and thetracking signal has an inverted polarity as that of the zero-orderlight. As a consequence, since a differential output is produced fromthese tracking signals, such a tracking signal from which the offset iscanceled may be obtained. In the case that the track pitch is madenarrow, since the adverse influence of the interference caused by thediffraction light is not present in the inner-sided luminous flux, theinner-sided luminous flux is arranged along an essentially radialdirection, the diffraction angle of the diffraction light can be easilysuppressed within the allowable range as to the view angle of theobjective lens. Based upon the tracking signal acquired in this manner,the tracking control mechanism is controlled. At the same time, thefocal shifts are detected as to the respective inner-sided andouter-sided luminous flux separated from the luminous flux. Then,spherical aberration is detected from a difference between thesedetected focal shifts, and also, a focal shift is detected from asummation of these detected focal shifts, so that the sphericalaberration applying mechanism and the focal shift control mechanism arecontrolled.

[0031] Alternatively, as another method, in the optical disk apparatushaving the spherical aberration control mechanism, after a focus controloperation has been commenced with respect to a recording layer on anoptical disk, a focal point position is moved from a focused positionalong forward/backward directions, and spherical aberration is detectedfrom a change in amplitudes of the tracking signal so as to drive thespherical aberration control mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a diagram for indicating a basic embodiment mode of anoptical disk apparatus according to the present invention;

[0033]FIG. 2 is a schematic diagram for representing a pattern of anoptical separating hologram employed in the embodiment mode of FIG. 1;

[0034]FIG. 3 is a diagram for showing a circuit calculation method and apattern of a light receiving plane of a photodetector 112 employed inthe embodiment mode of FIG. 1;

[0035]FIG. 4 is a diagram for explaining a principle detecting idea ofspherical aberration;

[0036]FIG. 5 is a diagram for representing a simulation of a separatedluminous flux focal shift signal of the present invention based uponspherical aberration;

[0037]FIG. 6 is a diagram for indicating a simulation of a sphericalaberration signal according to the present invention;

[0038]FIG. 7 is a diagram for representing the simulation of theseparated luminous flux focal shift signal of the prior art based uponspherical aberration;

[0039]FIG. 8 is a diagram for indicating the simulation of the sphericalaberration signal in the prior art;

[0040]FIG. 9 is a diagram for showing a spherical aberration signalaccording to the present invention;

[0041]FIG. 10 is a diagram for representing a model of a two-lenscalculation example using the prior art;

[0042]FIG. 11 is a diagram for showing the two-lens shape of the priorart;

[0043]FIG. 12 is a diagram for showing a change in spherical aberrationcaused by a two-lens interval;

[0044]FIG. 13 is a diagram for indicating an optical disk apparatusaccording to an embodiment mode in the case that two optical branchingelements are constructed in an integral form;

[0045]FIG. 14 is a diagram for showing an optical disk apparatusaccording to another embodiment mode of the present invention;

[0046]FIG. 15 is a diagram for explaining a diffraction grating of FIG.14;

[0047]FIG. 16 is a diagram for explaining a photodetector of FIG. 14;

[0048]FIG. 17 is a diagram for explaining a spherical aberrationcompensating system of FIG. 14;

[0049]FIG. 18 is a diagram for showing an optical disk apparatus inwhich a spherical aberration compensating mechanism compensates anaberration amount of incident light, according to an embodiment mode ofthe present invention;

[0050]FIG. 19 is a diagram for showing a control block of theabove-described embodiment mode;

[0051]FIG. 20 is a circuit diagram of a spherical aberration correctingcircuit employed in the above-described embodiment mode;

[0052]FIG. 21 is a diagram for showing a transfer frequencycharacteristic in a portion of a control system;

[0053]FIG. 22 is a diagram for representing a frequency characteristicof a phase lead compensating circuit;

[0054]FIG. 23 is a diagram for indicating an open-loop transfercharacteristic of the control system;

[0055]FIG. 24 is a diagram for indicating an optical disk apparatusaccording to an embodiment mode of the present invention in the casethat a non-polarization beam splitter is employed;

[0056]FIG. 25 is a diagram for indicating an optical disk apparatus,according to an embodiment mode, capable of suppressing an occurrence ofspherical aberration in an one-way optical path;

[0057]FIG. 26 is a diagram for showing a pattern of a polarizingdiffraction grating employed in the embodiment mode of FIG. 25;

[0058]FIG. 27 is a diagram for indicating a detector pattern and asignal calculation method employed in the embodiment mode of FIG. 25;

[0059]FIG. 28 is a diagram for representing an optical disk apparatusaccording to an embodiment mode of the present invention, in which aliquid crystal phase compensating element is mounted on an actuator;

[0060]FIG. 29 is a diagram for showing an optical disk apparatusequipped with a comma aberration compensating function, according to anembodiment mode of the present invention;

[0061]FIG. 30 is a structural diagram of a liquid crystal phasecompensating element;

[0062]FIG. 31 is a circuit diagram of a liquid crystal drive circuit;

[0063]FIG. 32 is a diagram for showing a compact optical head accordingto an embodiment mode of the present invention;

[0064]FIG. 33 is a diagram for indicating a relationship between aworking distance of an objective lens and an effective diameter of theobjective lens;

[0065]FIG. 34 is a perspective view of a laser module;

[0066]FIG. 35 is a diagram for representing a laser module detectorpattern and a signal calculating method;

[0067]FIG. 36 is a diagram for showing a polarizing diffraction gratingpattern used for a compact optical head;

[0068]FIG. 37 is a diagram for indicating an optical disk apparatusconstructed of the compact optical head, according to an embodiment modeof the present invention;

[0069]FIG. 38 is a diagram for indicating an optical disk apparatus withemployment of an optical disk having a narrow track pitch, according toan embodiment mode of the present invention;

[0070]FIG. 39 is a diagram for showing a diffraction grating pattern inthe embodiment mode of FIG. 38;

[0071]FIG. 40 is a schematic diagram for representing a spot arrangementon the optical disk in the embodiment mode of FIG. 38;

[0072]FIG. 41 is a diagram for indicating a photodetector lightreceiving pattern and a signal calculation method in the embodiment modeof FIG. 38;

[0073]FIG. 42 is a diagram for showing an optical disk apparatusaccording to an embodiment mode of the present invention, in which thediffraction grating of the embodiment mode of FIG. 38 is arranged in adetecting optical system;

[0074]FIG. 43 is a diagram for indicating a photodetector lightreceiving pattern and a signal calculating method in the embodiment modeof FIG. 42;

[0075]FIG. 44 is a diagram for indicating a photodetector lightreceiving pattern and a signal calculating method employed in anotherdetecting system in the embodiment mode of FIG. 38;

[0076]FIG. 45 is a diagram for showing a defocus characteristic of apush-pull signal in the case that spherical aberration is present;

[0077]FIG. 46 is a diagram for indicating a calculation result of aspherical aberration signal by a difference in the push-pull signalswhen a focus offset is applied;

[0078]FIG. 47 is a block diagram of a control compensation system;

[0079]FIG. 48 is a block diagram for showing a control compensationsystem into which a low-pass filter is inserted;

[0080]FIG. 49 is a block diagram for indicating a control system forcompensating spherical aberration as to only an one-way optical path;

[0081]FIG. 50 is a diagram for indicating a laser module according toanother embodiment mode of the present invention, employed in theembodiment mode of FIG. 32;

[0082]FIG. 51 is a diagram for showing a diffraction grating used in theembodiment mode of FIG. 50;

[0083]FIG. 52 is a diagram for indicating a calculation result of afocal shift signal by spherical aberration occurred on a detector, whichis caused by a thickness shift of a base plate;

[0084]FIG. 53 is a diagram for representing a focal shift signal offsetamount with respect to the spherical aberration in FIG. 52;

[0085]FIG. 54 is a diagram for indicating an optical disk apparatus,according to an embodiment mode of the present invention, capable ofcompensating a focal shift signal offset in the case that only sphericalaberration of an one way is corrected; and

[0086]FIG. 55 is a diagram for showing a calculation result of a focalshift signal after a focal shift signal offset in the case thatspherical aberration remains has been compensated.

BEST MODE FOR CARRYING OUT THE INVENTION

[0087] (Embodiment 1)

[0088] Referring now to drawing, an embodiment mode of the presentinvention will be described.

[0089]FIG. 1 schematically shows a basic embodiment mode of an opticaldisk apparatus according to the present invention.

[0090] Light emitted from a semiconductor laser 101 is collimated intoparallel light by a collimating lens 102, and this collimated lightpasses through a beam splitter 103, and then this collimated light iscondensed over a base plate onto a recording film plane of an opticaldisk 108 by a two-group/two-sheet of objective lenses 106 and 107. Thebeam splitter corresponds to a first optical branching element asrecited in a claim. In the two-group/two-sheet of objective lenses, afirst lens 106 is mounted on a two-dimensional actuator 104 and isdriven along both an optical axis direction and a radial direction ofthe optical disk. The second lens 107 is mounted on a sphericalaberration correcting actuator 105 which is driven in combination withthe first lens. While an interval between the two lenses is varied,spherical aberration is produced in response to this interval. Lightreflected from the optical disk 108 is reflected on the beam splitter103 to be entered into an optical separating hologram 109. Both light(not shown) located in the vicinity of an optical axis and light (notshown) located at a peripheral portion of the optical axis are separatedalong different directions, and both the separated light and light areentered via a cylindrical lens 111 into a photodetector 112 by acondenser lens 110. While the photodetector 112 owns a plurality oflight receiving regions, a plurality of the above-described light aresplit and detected by the plural light receiving regions so as to beconverted into optical currents. These optical currents are detected bya signal detecting circuit 113, a tracking error signal detectingcircuit 114, a spherical aberration signal detecting circuit 115, and areproduction signal detecting circuit 116 so as to be outputted asvoltage signals, respectively. A focal shift signal is fed back as adrive signal of the two-dimensional actuator 106 along a focal-pointdirection so as to execute a control operation in such a manner that anoptimum image point can be continuously focused onto the optical disk. Atracking error signal is fed back as a drive signal of thetwo-dimensional actuator 104 along a disk radial direction. A sphericalaberration signal is fed back to the spherical aberration correctingactuator 105 in order to perform a control operation in such a mannerthat spherical aberration caused by a fluctuation in thicknesses of abase plate and also by a lens interval shift can be compensated. Thereproduction signal detecting circuit 116 reproduces signals recorded onthe optical disk by involving a current-to-voltage conversion, awaveform equalizing process operation, a binary process operation, andthe like. In FIG. 1, the collimating lens 102 may be arranged betweenthe beam splitter 103 and the first lens 106 by being commonly used withthe condenser lens 109. Also, in order to improve a light utilizationefficiency, a ¼-wavelength plate is positioned between the beam splitter103 and the first lens 106, and this beam splitter 103 may be operatedas a polarization beam splitter. In this embodiment, as the focal shiftdetecting system, the cylindrical lens 111 is arranged in order toindicate such a case that an astigmatism system is employed. However,for example, when a knife edge system and a beam size system isemployed, the cylindrical lens 111 is no longer required. Also, in thecase of the astigmatism system, such an element capable of producingastigmatism may be employed. For instance, the cylindrical lens 111 maybe replaced by an inclined parallel flat plate. Also, in this embodimentmode, the spherical aberration signal is fed back to the interval of thetwo-group/two-sheet of objective lenses as the spherical aberrationcompensating mechanism. This compensating mechanism may be alternativelyrealized by mounting, for example, the collimating lens 102.Alternatively, while a liquid crystal variable phase modulating elementdriven by a voltage is employed, a wavefront may be directly modulated.

[0091] In FIG. 2, there is shown a schematic diagram of a pattern of theabove-described optical separating hologram 109 employed in theembodiment mode of FIG. 1. A boundary 202 having such a radius is set insuch a manner that light amounts of inner-sided/outer-sided regions maybe made substantially equal to each other with respect to a diameter ofan incident luminous flux 201. Within the inner-sided region 203 and theouter-sided region 204, directions of diffraction gratings are madedifferent from each other. As a result, an inner side of the luminousflux and an outer side thereof are separated from each other, and then,the separated luminous flux is condensed onto the detector 112. In thiscase, such an example by the astigmatism system has been exemplified.For instance, in the knife edge system, as to such a region that atleast one diameter for further subdividing the luminous flux by 2 isemployed as a further boundary, the directions of the diffractiongratings may be made different from each other. When the beam sizedetecting system is employed, while the directions of the diffractiongratings are made different from each other in both the inner side andthe outer side of FIG. 2, the diffraction grating may be replaced by acurved line grating.

[0092]FIG. 3 is a schematic diagram which shows both a light receivingplane pattern of the photodetector 112, and a circuit calculation methodin the embodiment mode of FIG. 1. This circuit calculation method mayobtain a focal shift signal, a spherical aberration signal, a trackingerror signal, and a reproduction signal from an output signal of thisphotodetector 112. The light which has been separated into both theinner side and the outer side of the luminous flux by the opticalseparating hologram 109 is received by four sets of light receivingregions 301, 302, 303, and 304. Among those light receiving regions,both outer-sided luminous flux first-order diffraction light 305 andinner-sided luminous flux first-order diffraction light 306 are receivedby the 4-split light receiving regions 301 and 302, whereas bothouter-sided luminous flux first-order diffraction light 307 andinner-sided luminous flux first-order light 308 are received by thenon-divided light receiving regions 303 and 304. Outputs of these lightreceiving regions are converted into voltages by a buffer amplifier 309.While using differential amplifiers 311, 312, 313, 314, 315, and 316 byproper gains which are determined by a resistor 310, these voltages areadded/subtracted from each other. At this time, as to both theinner-sided luminous flux and the outer-sided luminous flux, afteroutputs from two sets of diagonal regions are added to each other, theadded signals are subtracted from each other so as to obtain independentfocal shift signals. Thereafter, since the focal shift signal of theinner-sided region is added to the focal shift signal of the outer-sidedregion, focal shift signals are obtained, and then, these focal shiftsignals are substracted from each other, so that a spherical aberrationsignal may be obtained. At this time, in such a case that a light amountratio of the inner-sided subdivided region to the outer-sided subdividedregion of the luminous flux is not uniform, while the resistor 310 isreplaced by a variable resistor, the light amount ratio may be adjusted.A tracking error signal may be calculated in such a manner that whilethe push-pull system is employed, a difference is calculated betweendetection light amounts of light flux as to two regions which areobtained by subdividing the disk by the diameter of this disk along theradial direction thereof. In the normal push-pull system, a calculationis made of a difference in light receiving amounts of two regions whichare subdivided by a diameter along a tangential direction. However,since the astigmatism focal shift detecting system is employed in theembodiment mode of FIG. 1, the direction of the luminous flux is rotatedby 90 degrees in a least circle of confusion caused by the astigmatism,and then, a diffraction pattern caused by guide grooves of a diskappears along the tangential direction. As a consequence, thesubdivision is performed by the diameter along the radial direction.

[0093] Next, a detecting principle idea of spherical aberration will nowbe explained with reference to FIG. 4. When spherical aberration occurs,as indicated in this drawing, among light condensed by the lens 401, aposition of a focal point as to light located near an optical axis ismade different from a position of a focal point as to light located farfrom the optical axis. As a result, when the light is separated into aninner side and an outer side of a luminous flux, focal shift signals ofthe respective sides are shifted in connection with this positional siftof the focal points. As a consequence, a difference between the focalshift signal of the inner side of the luminous flux and the focal shiftsignal of the outer side thereof may represent spherical aberration.Also, in the above-described second prior art, since the luminous fluxis separated into both the inner side and the outer side on thephotodetector when the astigmatism focal shift is detected, thespherical aberration may be detected based upon the principle ideasimilar to that of the present invention. However, when the luminousflux is separated on the detector, in the case that the sphericalaberration is large, the light on the inner side of the luminous flux isoverlapped with the light on the outer side of the luminous flux, sothat the light on the inner side of the luminous flux cannot becompletely separated from the light on the outer side thereof, andfurthermore, the signal is deteriorated by interference caused by theoverlapped light. As a consequence, in accordance with the presentinvention, the luminous flux of the inner side is separated from theluminous flux of the outer side before being entered into the detector.

[0094] As will be explained with reference to drawings, a position ofseparating a luminous flux may be selected from several separationpoints, depending upon differences in effects. As a first separationpoint, a position between the first optical branching element and thephotodetector may be conceived. In this case, while such a diffractiongrating having no polarizing characteristic (namely, non-polarizationdiffraction grating) is employed, the luminous flux of the inner sidemay be preferably separated from the luminous flux of the outer side atdifferent photodetector positions. At this time, since the phase of thediffraction grating is properly selected, in such a case that the lightis not diffracted by 100%, but the zero-order light is lefted, suchlight which is not separated to the inner side and the outer side can bedetected at the same time. For instance, when it is so assumed that thislight corresponds to an RF signal, the light of the entire region of theluminous flux is detected without subdividing the photodetector. As aresult, it is possible to avoid that noise produced from plural sets ofcurrent/voltage converting amplifiers is mixed. Also, as a secondseparation position, such a position between the first optical branchingelement and the objective lenses may be conceived. In this case, apolarizing diffraction grating may be preferably employed. Then, forexample, since it is so arranged that the light is not diffracted in theluminous flux (luminous flux in going optical path) which is directed tothe optical disk and the light is diffracted in the luminous flux(luminous flux in returning optical path) which is reflected from theoptical disk to be returned, a loss in the light amount can besuppressed. As to the polarizing diffraction grating, compatibilitybetween such a condition that the light is not diffracted in theluminous flux of the going optical path, and another condition to makesuch a construction that the zero-order light is left in the luminousflux of the returning optical path can be hardly established due tomanufacturing aspect, and also precision of the film thickness can behardly made, as compared with the non-polarization diffraction grating.However, this compatibility is not impossible in principle. Furthermore,in this case, the following structure may be realized. That is, thelight is diffracted in the luminous flux of the going optical path,whereas the light is not diffracted in the luminous flux of thereturning optical path. At this time, optical spots of a plurality ofluminous flux to be separated are produced on the plane of the opticaldisk. If these optical spots are employed, then a tracking controlsignal may be obtained by a differential push-pull method (will beexplained later). This differential tracking method may become advantagein the case that a track pitch is narrower than a diameter of an opticalspot. Also, in addition, as a third separation position, a positionbetween the semiconductor laser and the first optical branching elementmay be conceived. In this case, since the luminous flux reflected fromthe disk does not again pass through this separating element, theluminous flux of the inner side may be separated from the luminous fluxof the outer side by employing the non-polarization diffraction grating.When the non-polarization diffraction gating is used, there are othermerits that the diffraction efficiency can be freely selected, and atthe same time, the cost of this non-polarization diffraction grating canbe made cheaper than that of the polarizing diffraction grating. In thiscase, a plurality of optical spots caused by the diffracted luminousflux are formed on the optical disk. As previously explained, also inthis case, there is a merit to employ the differential push-pull methodin the case that a track pitch is made narrower than a diameter of anoptical spot. As a fourth separation position, such a position may beconceived by that the first optical branching element is commonly used.In this case, there is a merit in such a case of a laser module in whichboth a semiconductor laser and a photodetector are mounted in the samepackage. In this case, a polarizing diffraction grating may bepreferably employed as the optical branching element. Since the firstoptical branching element is commonly used with the polarizingdiffractive grating, zero-order light which is not diffracted isreturned to the semiconductor laser. As a result, while the light is notdiffracted in the luminous flux of the going optical path, but isdiffracted in the luminous flux of the returning optical path, all ofsuch zero-order light may be diffracted without any remainder.

[0095] Also, since the luminous flux is separated to be detected, theaberration may be essentially reduced. Normally, as to aberration, anRMS value of wavefront aberration is directly employed as an evaluationindex. If luminous flux is subdivided to restrict the subdividedluminous flux, then RMS wavefront aberration in each of the subdividedluminous flux may be decreased. As a consequence, it can be expectedthat a deterioration of a focal shift signal may be reduced, and also,an offset may be mitigated. Also, a symbol of spherical aberration inthe below-mentioned description is defined as shown in drawings.

[0096]FIG. 5 represents a result obtained by that the principledetecting idea of spherical aberration according to the presentinvention could be confirmed by way of a simulation. This simulation wascarried out in such a way that a light intensity distribution on thedetector was calculated by the Fourier integrals based upon the scalardiffraction theory. The focal shift detecting system corresponds to theastigmatism method. As a calculation condition, a wavelength is 655 nm;a rim strength is 0.57; an NA of an objective lens is 0.6; an NA of acondenser lens in a detecting system is 0.088; astigmatism in thedetecting system is 0.92 mm; a size of a four-split photodetector is 100μm□; a width of a splitting line of the detector is 10 μm; and adiameter effective aperture ratio of a luminous flux boundary is 70.7%.An abscissa of the graph shows a focal shift amount of a spot on anoptical disk, and an ordinate of the graph indicates a focal shiftsignal which is normalized by an amplitude. FIG. 5(a) shows thatspherical aberration is expressed by −0.6λ by a wavefront aberrationcoefficient of Seidel. FIG. 5(b) indicates no aberration. FIG. 5(c)indicates such a case of +0.6λ, in which there are a signal producedonly from an inner side of luminous flux, a signal produced from onlyfrom an outer side of the luminous flux, and a signal produced bydetecting the entire sides of the luminous flux at the same time. It canbe seen that the focal shift signals of the inner/outer sides of theluminous flux are shifted.

[0097]FIG. 6 shows a calculation result obtained by calculating thespherical aberration signal in the present invention with employment ofthis result. In FIG. 6(a), an abscissa indicates a defocus amount on anoptical disk, and an ordinate represents a spherical aberration signalwhile spherical aberration is changed. It can be seen that while afocusing position is set as a center, a signal which is directlyproportional to spherical aberration is obtained within a range betweenapproximately +3 μm and approximately −3 μm. In FIG. 6(b), an abscissashows spherical aberration, and an ordinate indicates a sphericalaberration signal while a defocus amount is changed. When a defocusphenomenon appears, an offset is slightly applied to the sphericalaberration signal. However, it may be recognized that such a signal canbe detected which is directly proportional to the spherical aberrationunder substantially better condition.

[0098] In comparison with the present invention, FIG. 7 indicates aresult obtained by that an inner-side region and an outer-side region ofluminous flux are subdivided not in the luminous flux, but on a detectorin accordance with the second prior art, and then, focal shift signalsof the respective divided regions are calculated. It can be seen thatwaveforms of the focal shift signals are considerably deteriorated, ascompared with those of FIG. 5. In particular, when there is sphericalaberration, offsets in DC manners are produced in both the inner-sideregion and the outer-side region.

[0099]FIG. 8 shows a calculation result obtained in such a manner that acalculation similar to FIG. 6 is carried out based upon the prior art,while luminous flux is subdivided on the detector. It can be seen thatthe spherical aberration signal is rapidly changed with respect to thedefocusing phenomenon, as compared with that of FIG. 6. As a result, asshown in FIG. 6(b), it can also be understood that a sensitivity of asignal with respect to spherical aberration is rapidly lowered due tothe defocusing phenomenon, and a DC offset is increased.

[0100]FIG. 9 represents a calculation result of offsets of a focal shiftsignal with respect to spherical aberration. It can be seen that as toall luminous flux, an offset of a focal shift signal is increased due tospherical aberration. However, it can be seen that when an inner side ofthe luminous flux and an outer side of the luminous flux are separatedfrom each other to be detected in accordance with the present invention,an offset is extremely decreased.

[0101]FIG. 10 indicates a calculation model used to confirm acompensation effect of a two-sheet lens capable of compensating detectedspherical aberration by a lens interval. This is the lens shapeindicated in the above-described first prior art, and is atwo-group/two-sheet of objective lenses having a wavelength of 410 nm,and an NA of 0.85. A thickness of the disk base plate 108 is 0.1 mm.

[0102]FIG. 11 indicates a plane shape of this lens. Plane numbers aresequentially ordered from the left side of FIG. 10.

[0103]FIG. 12 shows a calculation result of spherical aberration whichoccurs when an interval between lenses is changed. An ordinate of thisdrawing shows a spherical aberration coefficient of Seidel which isexpressed in the unit of a wavelength. It can be seen that the sphericalaberration is changed based upon an interval between planes.

[0104] (Embodiment 2)

[0105]FIG. 13 shows on optical disk apparatus according to an embodimentof the present invention in the case that both a first optical branchingelement and a second optical branching element are formed in an integralform. In this embodiment, a semiconductor laser 1303, and both aphotodetector 1302 and another photodetector 1304 are constructed in asingle package 1301 in an integral form. The two optical branchingelements constitute a composite optical branching element 1305 in whichboth a ¼-wavelength plate and a polarizing diffraction grating areformed in an integral form. The composite optical branching element 1305is operated in such a manner that a polarizing diffraction gratingprovided on an incident side is not actuated as to polarized lightentered from the semiconductor laser, but this polarized light isconverted into circularly polarized light by a ¼-wavelength plateprovided on the projection side. Also, light reflected on the opticaldisk 108 is again entered into the ¼-wavelength plate so as to beconverted into linearly polarized light, the polarization direction ofwhich is rotated by 90 degrees, as compared with that when the light isemitted from the semiconductor laser, and then, this linearly polarizedlight is entered into the polarizing diffraction grating. At this time,a phase shift of the diffraction grating is effected to the linearlypolarized light, so that this linearly polarized light is diffracted.Then, the diffracted light is condensed to the photodetectors 1302 and1304 by the collimating lens 102. As previously indicated, as to apattern of the polarizing diffraction grating, in the case that theastigmatism system is employed as the focal shift detecting system, acurved line diffraction grating may be employed which diffracts lightalong a direction of a detector and at the same time, causesastigmatism. As a light receiving plane pattern of the photodetector,the semiconductor laser 1303 may be arranged in such a manner that alight emitting point of a laser is positioned at a center of FIG. 3. Forexample, if silicon is employed as a substrate of a photodetector, thena mirror inclined at 45 degrees can be readily formed by way of ananisotropic etching. As a result, if laser light emitted from asemiconductor laser is raised by employing this mirror, then both thesemiconductor laser and the photodetector may be made compact in anintegral body by merely arranging the photodetector at a peripheralportion of the semiconductor laser.

[0106] (Embodiment 3)

[0107]FIG. 14 shows an optical disk apparatus according to a furtherenbodiment mode. While light emitted from a semiconductor laser 1401 iscollimated by a collimating lens 1402 into parallel light and also anellipse-shaped beam of an intensity distribution is converted into acircular beam by beam forming prisms 1403 and 1404, spherical aberrationis added by a liquid crystal phase compensating element 1405. The liquidcrystal phase compensating element 1405 corresponds to such acompensating element that liquid crystal is sandwiched by two baseplates on which transparent electrodes are patterned, and a phase ofpenetration light can be changed by applying an AC voltage to thetransparent electrodes. The transparent electrodes are separated into aplurality of regions in correspondence with a wavefront shape ofspherical aberration, and voltages are applied in such a manner thatphase differences are lowered in the respective regions. The light whichhas passed through the liquid crystal is penetrated through apolarization beam splitter 1406, a ¼-λ (wavelength) plate 1407, araising mirror 1408, and an objective lens 1411, and then, is condensedonto an optical disk 1412. The objective lens 1411 is mounted on anactuator 1410 so as to perform a focus control operation and a trackingcontrol operation. Light reflected from the optical disk 1412 isreturned via the same optical path up to the polarization beam splitter1406, and then, this reflection light is reflected on this polarizationbeam splitter 1406 to be entered into the ½-λ plate 1413. This ½-λ plate1413 is employed in the case that a separation ratio of luminous flux atthe next polarization beam splitter 1414 is controlled by rotating the½-λ plate 1413 around the optical axis thereof. The light which haspassed through the polarization beam splitter 1414 is used to detect atracking signal by the diffraction grating 1415 in such a way that theluminous flux is subdivided into four sets of this luminous flux bydiameters thereof along a radial direction and a tangential direction,and then, the four-divided luminous flux is entered into thephotodetector 1417. Since output signals of this photodetector 1417 arecalculated by a tracking servo circuit 1422 and a reproduction signalcircuit 1423, both a tracking signal and a reproduced RF signal aredetected. The tracking signal is fed back to the actuator 1410 incombination with a focal shift signal (will be explained later). On theother hand, the light reflected on the polarization beam splitter 1414is reflected by a reflection mirror 1418, luminous flux is subdivided bya diffraction grating 1419, and then, the subdivided luminous flux iscondensed onto a photodetector 1421 by a condenser lens 1420. Based uponoutput signals from the photdetector 1421, a focal shift signal isdetected by an AF servo circuit and a spherical aberration signal isdetected by a spherical aberration servo circuit. Then, the focal shiftsignal is fed back to the actuator 1410, and the spherical aberrationsignal is fed back to the liquid crystal phase compensating element1405. In this case, different from the previous embodiment mode, as tosuch a case that the knife edge method is employed so as to detect afocal point, both structures of the diffraction grating 1419 and of thephotodetector 1421 will now be explained.

[0108]FIG. 15 shows an embodiment mode of the diffraction grating 1419.This diffraction grating 1419 separates luminous flux into an inner sideand an outer side of the luminous flux, and at the same time, subdividesluminous flux reflected from an optical disk as to a diameter of aradial direction of the optical disk so as to detect the inner-sidedluminous flux and the outer-sided luminous flux, separately, so thatthis diffraction grating may independently detect a focal point errorsignal caused by the inner-sided luminous flux, and another focal pointerror signal caused by the outer-sided luminous flux.

[0109]FIG. 16 represents the photodetector 1421 and diffraction lightwhich is entered into the photodetector 1421 and is diffracted by thediffraction grating 141. For the sake of convenience, the diffractionlight indicates such a case that a focal shift is present, andzero-order light is not indicated in this drawing. In order thatsubstantially no zero-order light is produced, a depth of gratings ofthe diffraction grating 1419 may be easily adjusted. Alternatively,while such a zero-order light is produced, the light receiving portionis located also at a center portion, and thus, a total light amount maybe detected. In this case, the diffraction light is received byfour-split optical detecting regions 1601, 1602, and also two-splitoptical detecting regions 1603, 1604. Since output signals derived fromthe four-split optical detecting regions 1601 and 1602 are detected insuch a manner as shown in this drawing, and these detected signals arecalculated in such a manner as shown in a lower portion of this drawing,a spherical aberration signal SAS may be detected. In order tocompensate for adverse influences caused by a fluctuation in intensitydistributions of the semiconductor laser, the inner-sided focal shiftsignal is multiplied by a gain “G” in a subtraction calculation betweenthe outer-sided focal shift signal and the inner-sided focal shiftsignal. A focal shift signal “FES” may be obtained by similarlycalculating output signals derived from the two-split optical detectingregions 1603 and 1604 in a calculation manner as shown in this drawing.In this case, the separate diffraction light has been employed.Alternatively, as apparent from the foregoing description, the focalshift signal may be obtained by adding the inner-sided focal shiftsignal to the outer-sided focal shift signal. In the above-describeddetecting system, with employment of the knife edge method, in the casethat an optical spot on the disk plane is brought into the focusingcondition, an optical spot on the photodetector is also focused. As aresult, in the case that this detecting system is applied to, forexample, a two-layer disk and the like, a size of a light receivingplane is designed to be properly a small size, so that a crosstalkphenomenon caused by another layer can be reduced.

[0110] In this embodiment mode shown in FIG. 14, since the liquidcrystal phase compensating element is effected only in the optical pathof the light directed to the optical disk, the spherical aberration tobe compensated does not give the compensating effect only in one way.However, the spherical aberration which is caused by the thickness errorof the base plate of the optical disk will occur not only in theincident light, but also in the reflection light. As a result, thespherical aberration which is detected by the photodetector may occur inthe reciprocative optical path. As a consequence, when the controloperation is carried out in such a manner that the liquid crystal phasecompensating element is directly driven by the spherical aberrationsignal so as to reduce the spherical aberration of the detected luminousflux to zero, there is such a problem that the spherical aberration ofthe focal point on the disk plane is excessively corrected. This problemmay similarly occur even when the liquid crystal phase compensatingelement is interposed between the polarization beam splitter 1406 andthe objective lens 1411. This is because the phase difference isnormally added only by the linearly polarized light along a specificdirection in the liquid crystal phase compensating element. In such acase that optical isolation achieved by the ¼-λ plate 1407 and thepolarization beam splitter 1406 is not employed, the phases may be addedin the reciprocative luminous flux by the liquid crystal phasecompensating element. However, an optical utilization efficiency may belowered. In such a case, a schematic diagram of a control system isshown in FIG. 17, while this control system may compensate for sphericalaberration of an optical spot on a focal plane. In this drawing, anoptical system is indicated in a simple manner. In such a case thatspherical aberration W′ is effected by the liquid crystal phasecompensating element 1405, and spherical aberration W of a going opticalpath is effected due to a thickness shift of a base plate of an opticaldisk, spherical aberration of a focal point on a plane of the opticaldisk becomes W−W′=δ. Then, since spherical aberration is furthermoreeffected also to luminous flux of a returning optical path, sphericalaberration of luminous flux at a position of the photodetector 1421becomes 2W−W′=δ+W. It is so assumed that this spherical deviation isdetected by the detecting system as such a spherical aberration signalof SAS=α(δ+W). In this formula, symbol “α” shows a gain of the detectingsystem, and is normally assumed as a negative gain due to a feedbackcontrol operation. This signal is inputted into a differential amplifier1701, and a difference signal between the previous signal and thefeedback signal is furthermore amplified by a multiplication factor of“k” by an amplifier 1702. This signal is amplified by a multiplicationfactor of “γ” by an amplifier 1703, and the amplified signal is used asthe previous feedback signal, and at the same time, is entered into aliquid crystal drive circuit 1704. Thus, spherical aberration −W′ whichis added by the previously-shown liquid crystal phase compensatingelement 1405 is equal to a value obtained by multiplying the inputsignal to the liquid crystal drive circuit by “β”. At this time, thebelow-mentioned expression 1 can be established: $\begin{matrix}{{k\left( {{SAS} - {\gamma \quad \frac{- W^{\prime}}{\beta}}} \right)} = \frac{- W^{\prime}}{\beta}} & \text{(Expression~~1)}\end{matrix}$

[0111] As a result, the following expression 2 may be conducted:$\begin{matrix}{\delta = {{W - W^{\prime}} = {\frac{1 + {k\left( {{\alpha\beta} + \gamma} \right)}}{1 - {k\left( {{\alpha\beta} - \gamma} \right)}}W}}} & \text{(Expression~~2)}\end{matrix}$

[0112] As a consequence, since the gain “γ” is controlled in such amanner that the following expression 3 may be established:

γ=−αβ  (Expression 3),

[0113] the below-mentioned expression 4 may be obtained: $\begin{matrix}{\delta = \left. {\frac{1}{1 - {2k\quad \alpha \quad \beta}}W}\rightarrow{0\left( k\rightarrow\infty \right)} \right.} & \text{(Expression~~4)}\end{matrix}$

[0114] When the gain “k” is sufficiently large, the spherical aberration“δ” on the disk plane can be approximated to zero.

[0115] (Embodiment 4)

[0116]FIG. 18 is an optical disk apparatus according to anotherembodiment mode of the present invention.

[0117] Light emitted from a semiconductor laser 101 is collimated intoparallel light by a collimating lens 102, and this collimated lightpasses through a beam splitter 103 via spherical aberration correctingactuator 1801 and then this collimated light is condensed over a baseplate onto a recording film plane of an optical disk 108 by atwo-group/two-sheet of objective lenses 106 and 107. The beam splittercorresponds to a first optical branching element as recited in a claim.In the two-group/two-sheet of objective lenses, a first lens 106 ismounted on a two-dimensional actuator 104 and is driven along both anoptical axis direction and a radial direction of the optical disk. Thesecond lens 107 is driven in combination with the first lens in anintegral form. Light reflected from the optical disk 108 is reflected onthe beam splitter 103 to be entered into an optical separating hologram109. Both light (not shown) located in the vicinity of an optical axisand light (not shown) located at a peripheral portion of the opticalaxis are separated along different directions, and both the separatedlight and light as entered via a cylindrical lens 111 into aphotodetector 112 by a condenser lens 110. While the photodetector 112owns a plurality of light receiving regions, a plurality of theabove-described light are subdivided and detected by the plural lightreceiving regions so as to be converted into optical currents. Theseoptical currents are detected by a signal detecting circuit 113, atracking error signal detecting circuit 114, a spherical aberrationsignal detecting circuit 1802, and a reproduction signal detectingcircuit 116 so as to be outputted as voltage signals, respectively. Afocal shift signal is fed back as a drive signal of the two-dimensionalactuator 106 along a focal-point direction so as to execute a controloperation in such a manner that an optimum image point can becontinuously focused onto the optical disk. A tracking error signal isfed back as a drive signal of the two-dimensional actuator 104 along adisk radial direction. A spherical aberration signal is fed back to thespherical aberration correcting actuator 1801 in order to perform acontrol operation in such a manner that spherical aberration caused by afluctuation in thicknesses of a base plate and also by a lens intervalshift can be compensated. The reproduction signal detecting circuit 116reproduces signals recorded on the optical disk by involving acurrent-to-voltage conversion, a waveform equalizing process operation,a binary process operation, and the like. In FIG. 18, the collimatinglens 102 may be arranged between the beam splitter 103 and the firstlens 106 by being commonly used with the condenser lens 109. Also, inorder to improve a light utilization efficiency, a ¼-wavelength plate ispositioned between the beam splitter 103 and the first lens 106, andthis beam splitter 103 may be operated as a polarization beam splitter.In this embodiment, as the focal shift detecting system, the cylindricallens 111 is arranged in order to indicate such a case that anastigmatism system is employed. However, for example, when a knife edgesystem and a beam size system is employed, the cylindrical lens 111 isno longer required. Also, in the case of the astigmatism system, such anelement capable of producing astigmatism may be employed. For instance,the cylindrical lens 111 may be replaced by an inclined parallel flatplate. Also, in this embodiment mode, the spherical aberration signal isfed back to the interval of the two-group/two-sheet of objective lensesas the spherical aberration compensating mechanism. This compensatingmechanism may be alternatively realized by mounting, for example, thecollimating lens 102. Alternatively, while a liquid crystal variablephase modulating element driven by a voltage is employed, a wavefrontmay be directly modulated. In this embodiment mode, since the sphericalaberration correction is entered only in the incident light, theabove-described control system may have the below-mentioned problem. Inthis control system, the spherical aberration is detected from thereflection light, and this detected spherical aberration is directly fedback.

[0118] It is now assumed that a spherical aberration error detected inthis arrangement is “ε”. While wavefront aberration of only “y” isapplied to incident luminous flux by a spherical aberration compensatingmechanism, this incident luminous flux is entered into a disk plane.Assuming now that spherical aberration occurred on the disk plane is“x”, aberration occurred on the disk plane becomes 2x, as viewed by thereflection light, but the aberration applied by the spherical aberrationcompensating mechanism directly remains. As a result, if the controloperation is carried out in such a manner that the detection error “ε”of the spherical aberration detecting system becomes zero, then y=2x,and over compensation by “x” is made on the disk plane. If such acontrol operation by which x=y is not carried out, then the sphericalaberration occurred on the disk plane cannot become zero.

[0119] As a consequence, a block arrangement of such a control system asshown in FIG. 19 is conceived.

[0120] A block 1901 indicates that aberration occurred in a reflectionstage is multiplied by 2. A block 1902 shows a spherical aberrationdetector which outputs the aberration error “ε”. This control system isbasically constituted by an aberration detector 1902, a controlcompensating system 1904, and a spherical aberration actuator 1905. Acontrol amount corresponds to the spherical aberration “y” of theincident light, and is driven by the spherical aberration compensationactuator 1905. Although a target value “x” cannot be directly measured,a deviation amount of 2x−y can be measured. As a result, such a feedbackcontrol system that “x−y” becomes zero is constituted by employing thiscontrol system. A different point of this control system from the normalcontrol system is such that the control compensating system 1904 isinserted. As a consequence, an open-loop transfer function “G” definedfrom the phase detecting unit up to the spherical aberrationcompensating actuator is given as follows:

G=G ₁ G ₂ G ₃  (Expression 5)

[0121] Also, a close-loop transfer function “H” may be expressed by thefollowing expression 6: $\begin{matrix}{H = \frac{G_{1}}{1 + G_{1}}} & \text{(Expression~~6)}\end{matrix}$

[0122] Furthermore, a transfer function to “y” viewed from “x” becomes2·H, and may be expressed by the below-mentioned expression 7:

y/x=2H  (Expression 7) $\begin{matrix}{{x - y} = {\frac{G - 1}{G + 1}x}} & \text{(Expression~~8)}\end{matrix}$

[0123] In order to control “x−y” to zero, the below-mentioned equationmust be satisfied within a range capable of involving “x”.

G=1  (Expression 9)

[0124] In other words,

G ₂=1/G ₁ G ₃  (Expression 10)

[0125] As a consequence, G2 may be selected in order to satisfy thecondition of the above-described expression (9).

[0126] As G2, the below-mentioned embodiments may be conceived.

[0127] (1) A product as to inverse functions of G1 and G3 is formed inaccordance with an expression “x”.

[0128] For instance, when a liquid crystal plate is used as thespherical aberration compensating actuator, a frequency characteristicof a wavefront phase amount with respect to a drive input representssuch a characteristic of a low-pass filter as shown in FIG. 46. As aconsequence, G3 is expressed by the below-mentioned transfer function:$\begin{matrix}{G_{3} = \frac{K_{3}}{{s\quad T} + 1}} & \text{(Expression~~11)}\end{matrix}$

[0129] G1 may be regarded as “k1”, namely substantially constant withina subject range. As a consequence, G2 may become the below-mentionedtransfer function: $\begin{matrix}{G_{2} = \frac{{s\quad T} + 1}{K_{1}K_{3}}} & \text{(Expression~~12)}\end{matrix}$

[0130] In other words, G2 may become such a transfer function made of asummation constituted by a transfer function having a gain of an inversenumber of a product between k1 and k2, another transfer function of adifferential of a time constant “T”, and also a constant 1. G2 may beexpressed as shown in FIG. 47. Since the differential is involved inthis control system a low-pass filter whose gain is lowered from afrequency higher than a control range may be interposed between G1 andG3 in order to arrange an actual control system in such a way that noiseis not increased in a high range, and further, this noise gives noadverse influence to the control system. Preferably, the above-describedlow-pass filter may be inserted after a differential circuit as shown inFIG. 48.

[0131] (2) In this embodiment, such a system capable of reducing theaberration detecting error to zero is considered as a basic system. Thisbasic system is arranged by an aberration detector 1902, a phasecompensating element 4902, an amplifier 4901, and a spherical aberrationcompensating actuator 1905. A control amount corresponds to thespherical aberration “y” of the incident light, and is directlyproportional to an input of the spherical aberration compensatingactuator 1905. Although the target value “x” cannot be directlymeasured, such a deviation amount of “2x−y” can be measured. As aresult, if the deviation amount is fed back in a similar manner to theprior art, then it becomes 2x=y.

[0132] Therefore, motion of the spherical aberration compensatingactuator 1905 is electrically simulated in a block 4903, the sphericalaberration simulated in a block 4904 is converted into an electricsignal, and then, this electric signal is subtracted from an aberrationdetection signal. As a consequence, the resultant signal obtained bysubtracting the electric signal from the aberration detection signalbecomes such a signal which is directly proportional to (2x−y)−y=2(x−y).When this signal is fed back, such a control operation that x=y may berealized. A block diagram capable of realizing this control operation isindicated in FIG. 49. As a transfer function of the block 4903, it maybe set as G3, and as a transfer function of the block 4904, it may beset as G1. Assuming now that the transfer function of the phasecompensating element 4902 is “g1”, and also, the transfer function ofthe amplifier 4901 is set to “g2” in this structure, a transfer functionG2 may be expressed as follows: $\begin{matrix}{G_{2} = {\frac{1}{{G_{1}G_{3}} + \frac{1}{g_{1}g_{2}}}.}} & \text{(Expression~~13)}\end{matrix}$

[0133] When (g1×g2) is sufficiently larger than 1, the transfer functionG2 may be given as follows:

G ₂=1/G ₁ G ₃  (Expression 14)

[0134] Accordingly, the transfer function G2 becomes an inverse functionof a product between G1 and G3.

[0135] To construct the above-described control system, a block of G2may be arranged as shown in FIG. 20. That is, while a sphericalaberration signal is entered into a plus terminal of a differentialamplifier 2005, a drive voltage of a drive circuit 2002 for driving aspherical aberration compensating actuator is inputted into a minusterminal thereof by such a signal which is penetrated through simulationcircuit 2001 having such a transfer function that a transfer function ofthe spherical aberration compensating actuator (containingcharacteristic of actuator drive circuit) is series-connected to atransfer function of an aberration detector. An output of thedifferential amplifier 2005 is entered to an amplifier 2004, and anoutput of this amplifier 2004 is entered via a secondary integratingcircuit 2006 to a phase lead compensating circuit 2003. Then, an outputof this phase lead compensating circuit 2003 may become the drivecircuit 2002 which drives the spherical aberration compensating actuator1905. The output of the drive circuit 2002 is entered into the sphericalaberration compensating actuator 1905, so that the spherical aberrationmay be applied to the incident light.

[0136] Next, a description will now be made of a method for designing acontrol system, while the structure of the above-described controlsystem (2) is exemplified. The phase compensating circuit 4902 isconstructed of both the secondary integrating circuit 2006 and the phaselead circuit 2003. The secondary integrating circuit owns such afrequency characteristic that a gain thereof is decreased by −40 dB/decwith respect to a frequency. A characteristic of the secondaryintegrating circuit is expressed as K/(s²). In this case, s=jω mayconstitute such a frequency characteristic as indicated in FIG. 21.

[0137] Furthermore, in order to improve a response characteristic of thecontrol system, a phase lead/delay circuit having a frequencycharacteristic represented in FIG. 22 is inserted. The frequencycompensation by the phase lead circuit may cause a servo system to bebrought into a stable condition. In order that the servo system becomesstable, in accordance with a simple stability judging method of Nyquist,a phase must be larger than, or equal to −180 degrees in such afrequency “ωc” (cross frequency) that the gain of the open-loop transferfunction (namely, open-loop transfer function is equal to G1G2G3 in thiscase) becomes 0 dB. If the phase is delayed by 180 degrees, then thecontrol system will oscillate.

[0138] As a consequence, how degree the phase at the cross frequency“ωc” is lead from −180 degrees may constitute an evaluation amount of astability. As a consequency, in the control system, the servo system isstabilized in such a way that in the phase lead circuit having thefrequency characteristic as indicated in FIG. 22, the phase at the crossfrequency “ωc” is led from −180 degrees and a phase margin is increasedby 40 to 50 degrees. In the case that α=0.1, the gain characteristic ofthe phase lead compensating circuit is increased by 20 dB in the highfrequency range.

[0139] An open-loop characteristic obtained after the phase leadcompensating circuit has been assembled is indicated in FIG. 23. In thisembodiment, the cross frequency is selected to be 1.7 KHz.

[0140] (Embodiment 5)

[0141]FIG. 24 is an embodiment of an optical disk apparatus in such acase that a non-polarization beam splitter 2401 is employed. Since thenon-polarization beam splitter 2401 is employed, even in such a casethat the liquid crystal phase compensating element 1405 is employed, ifthis liquid crystal phase compensating element 1405 is interposedbetween the beam splitter 2401 and the objective lens 1411, thenspherical aberration may effect both luminous flux of a going opticalpath and luminous flux of a returning optical path. As a result, since adetected spherical aberration signal may be obtained as a value which isdirectly proportional to spherical aberration occurred on the diskplane, a feedback control circuit may employ a spherical aberrationcontrol circuit 2402 which is constituted by an amplifier 1702 and aliquid crystal drive circuit 1704 and is similar to that of the priorart.

[0142]FIG. 25 is an embodiment mode of an optical disk apparatus in thecase that a lens by which spherical aberration may probably occur is notarranged in an optical path through which light will pass only in oneway, for instance, such a lens is not arranged in an optical path from asemiconductor laser up to an optical branching element, and in anotheroptical path from the optical branching element up to a photodetector.Light emitted from the semiconductor laser 101 is penetrated through thepolarization beam splitter 1406, and then, is collimated to obtainparallel light by the collimating lens 102, and spherical aberration isapplied to this collimated light by the liquid crystal phasecompensating element 1405. Furthermore, this light passes through boththe polarization diffraction grating 2501 and the ¼-λ plate 1407, whichare mounted on the objective lens actuator 104, and then, is condensedon the optical disk 108 by the objective lens 1411. The reflected lightis processed by the ¼-λ plate 1407 to be converted into linearlypolarized light which is located perpendicular to that when the light isentered, and then, this learly polarized light is diffracted by thepolarization diffraction grating 2501. While this diffracted lightpasses through the liquid crystal phase compensating element 1405 and iscondensed by the collimate lens 102, this condensed light is reflectedon the polarization beam splitter 1406 and then the reflected light isreceived by the photodetector 2502. When the optical disk apparatus isarranged in this manner, since the light necessarily passes through suchan optical component as a lens in the reciprocative optical path, whichmay probably cause the spherical aberration, an occurrence of offset canbe prevented in such a system that when the spherical aberration iscompensated, the aberration applied by the liquid crystal phasecompensating element occurs only in one optical path.

[0143]FIG. 26 is a diagram for indicating a pattern of the polarizationdiffraction grating 2501 employed in this optical disk apparatus. Thispattern constitutes such a grating pattern that astigmatism having thesame dimensions and directed to 45 degrees is produced in diffractedlight by both luminous flux located in the vicinity of an optical axisand luminous flux located at a peripheral portion of this optical axis,and at the same time, the diffracted light is separated along right/leftdirections in the vicinity of the optical axis and the diffracted lightis separated along upper/lower directions at the peripheral portion ofthe optical axis.

[0144]FIG. 27 shows the photodetector 2503, patterns of luminous fluxwhich is entered into this photodetector 2503, and calculation formulaeof respective signals. In a light receiving portion “A” of a centerportion, rays of zero-order light are entered which are not diffractedby the polarizing diffraction grating 2501. At this time, when a spot onthe optical disk is focused, the position of the photodetector 2502 isadjusted in such a manner that this zero-order light may be focused onthe light receiving portion A. At this time, although the diffractedlight is not condensed by the astigmatism but is widened, since the lenspower of condensing/diverganting operations is not present in thepattern of the polarizing diffractive grating 2501 of FIG. 26, a leastcircle of confusion is formed on the photodetector 2502. Thus, each ofthese diffracted light is received by a four-split optical detectingregion, and such a calculation is carried out so as to detect a focalpoint by subtracting summed outputs along a diagonal direction from eachother, and then, the subtracted values are added to each other, so thata focal shift signal (AF) can be acquired. It should be noted that as tothe astigmatism of the diffraction grating, since symbols are invertedin + first-order diffraction light and − first-order diffraction lightor the focal shift signals are added by considering polarities thereof.Since the least circle of confusion caused by the astigmatism along the45-degree direction becomes such a distribution that the distribution inthe parallel luminous flux is rotated by essentially 90 degrees, thediffraction pattern caused by the guide groove of the optical diskappears along the tangential direction. In this drawing, in order toindicate the polarity of the diffraction pattern, such an example isexemplified in which only a single side of the diffraction patternbecomes dark by supposing a condition that a slight tracking deviationis present. In this example, for example, assuming now that a pitch ofguide grooves of the optical disk is 0.32 μm; NA of the objective lensis 0.85; and a wavelength of light is 0.4 μm, since the pitch isnarrower than a spot diameter (λ/NA=0.47 μm), such a region having nodiffraction pattern caused by the guide grooves is present in a centerportion of luminous flux. As a result, when a tracking signalcalculation in such a region is carried out, this tracking signal mayconstitute a lens shift signal (LS), while the luminous flux located inthe vicinity of the optical axis is set in this region where these is nodiffraction pattern caused by the guide grooves. This lens shift signal(LS) is multiplied by a proper coefficient, and the multiplied lensshift signal is subtracted from a tracking signal of outer-sidedluminous flux (TR), so that this may solve the offset problem caused bythe objective lens shift which may cause the problem in the trackingsignal calculation of the push-pull system. As previously described, thespherical aberration may be detected in such a way that both the focalshift signal of the outer-sided luminous flux and the focal shift signalof the inner-sided luminous flux are acquired, and then, these focalshift signals are subtracted from each other (SA).

[0145] (Embodiment 6)

[0146]FIG. 28 is an embodiment mode of an optical disk apparatus in thecase that the liquid crystal phase compensating element 1405 is mountedon the two-dimensional actuator 104 in the above-described embodimentmode of FIG. 25. In this embodiment mode shown in FIG. 25, since theobjective lens 1411 is driven by the two-dimensional actuator 104independent from the liquid crystal phase compensating element 1405, thespherical aberration applied by the liquid crystal phase compensatingelement 1405 is shifted from the optical axis of the objective lens 1411by the drive amount by the two-dimensional actuator 104. The sphericalaberration shifted from the optical axis by “Δ” is expressed by thefollowing expression 15: $\begin{matrix}\begin{matrix}{{W(\Delta)} = \quad {W_{40}\left\{ {\left( {x - \Delta} \right)^{2} + y^{2}} \right\}^{2}}} \\{\cong \quad {W_{40}\left\{ {x^{2} + y^{2} - {2\Delta \quad x}} \right\}^{2}}} \\{= \quad {W_{40}\left\{ {\rho^{2} - {2\Delta \quad \rho \quad \cos \quad \theta}} \right\}^{2}}} \\{\cong \quad {W_{40}\left\{ {\rho^{4} - {2\quad \Delta \quad \rho^{3}\cos \quad \theta}} \right\}}}\end{matrix} & \text{(Expression~~15)}\end{matrix}$

[0147] As a result, comma aberration may occur which is approximatelyproportional to “Δ” in addition to the original spherical aberration. Ifthis comma aberration is present within an allowable range, then thereis no specific problem. In the case that a spherical aberration amountto be compensated is large, or the shift “Δ” is large and the occurringcomma aberration exceeds the allowable range, as represented in FIG. 28,since the liquid crystal phase compensating element 1405 is mounted onthe objective lens 1417, the occurrence of the comma aberration can besuppressed.

[0148] (Embodiment 7)

[0149] Also, as another method of suppressing an occurrence of commaaberration, as embodiment mode of an optical disk apparatus equippedwith a comma aberration compensating function is shown FIG. 29. In thisembodiment mode, as the liquid crystal phase compensating element, sucha liquid crystal phase compensating element 2901 capable of compensatingspherical aberration and at the same time capable of compensating commaaberration is employed. Either comma aberration or a lens shift, whichare produced in such a case that the objective lens 1411 is shifted froman axis of the liquid crystal phase compensating element 2901 by the twodimensional actuator 104 in conjunction with eccentricity of the opticaldisk 108, is detected from an output of a photodetector 2502 by way of acalculation by employing a comma aberration circuit 2902. Then, thedetected comma aberration, or the detected lens shift is fed back to acomma aberration drive electrode of the liquid crystal phasecompensating element 2901. At this time, as this photodetector 2502,such a photodetector as explained in FIG. 27 may be employed. Also, theliquid crystal phase compensating element 2901 may be alternativelyarranged between the semiconductor laser 101 and the beam splitter 2901by designing the collimating lens while considering such a sphericalaberration occurred when divergent light is entered thereinto. Also, inthe case that the non-polarization beam splitter as shown in FIG. 24 isemployed, while the arranging position of the liquid crystal phasecompensating element 2901 remains as shown in this drawing, thepolarizing diffraction grating 2501 may be replaced by such anon-polarizing diffraction grating, and the ¼-λ plate 1407 may beeliminated.

[0150] In this case, such a liquid crystal phase compensating elementcapable of simultaneously compensating both comma aberration andspherical aberration is indicated in FIG. 30. Such a liquid crystalphase compensating element is described in, for instance, JapaneseLaid-open Patent Application No. HEI 2001-84631. FIG. 30(a) shows asectional structure of this element. In this drawing, transparentelectrodes 3004 a and 3004 b are patterned on surfaces of glass baseplates 3001 a and 3001 b. Furthermore, insulating films 3005 a, 3005 b,and orientation films 3006 a, 3006 b are stacked on these surfaces, andliquid crystal 3003 is sandwiched by the stacked layer, which is tightlysealed by employing a sealing member 3002. The electrode 3004 b may bewired from the base plate 3001 a via a conductive film which ispatterned as the sealing member. FIG. 30(b) is a plan view forindicating the transparent electrode 3004 a for compensating thespherical aberration and FIG. 30(c) is a plan view for showing thetransparent electrode 3004 b for compensating the comma aberration. Asindicated in FIG. 30(d) and FIG. 30(e), AC voltages are applied to inputvoltages V1, V2, V3, and V4 in order to reduce the aberration as shownby wavefronts 3008 a and 3008 b by adding phase shifts with respect towavefronts 3007 a and 3007 b having aberration in a segment shape.

[0151]FIG. 31 shows a drive circuit for applying a voltage to the liquidcrystal of FIG. 30. While an AC voltage 3100 having a rectangularwaveform of +V0 and −V0, and an invented waveform thereof are used as areference application voltage for the spherical aberration and also areference application voltage for the comma aberration, amplitudes ofthese application voltages are modulated by a spherical aberrationsignal and a comma aberration signal. The amplitudes of these inputvoltages V1 and V2, and the amplitudes of the input voltages V3 and V4are increased, or decreased in an asymmetrical manner by the sphericalaberration signal and the comma aberration signal with respect to thereference voltages, respectively. Since the above-described voltagemodulations are carried out, such compensated wavefronts as indicated inFIG. 30(d) and FIG. 30(e) can be continously realized.

[0152]FIG. 30 and FIG. 31 indicate the embodiment mode of the liquidcrystal phase compensating elements which are used to simultaneouslycompensate both the spherical aberration and the comma aberration. Insuch a case that only the spherical aberration is compensated, while thetransparent electrode for compensating the comma aberration may be madeas an uniform electrode having no split, as the drive voltage, eitherthe ground level may be used, or an invented waveform of a referencesignal having a constant amplitude may be employed.

[0153]FIG. 32 shows an optical head according to an embodiment mode ofthe present invention, in which an effective luminous flux system of anobjective lens is smaller than, or equal to 1 mm; and a semiconductorlaser, a spherical aberration adding mechanism, an optical branchingelement, an objective lens, and a photodetector are arranged in anintegral form. A semiconductor laser chip 3201 is mounted on a substrate3202 of a photodetector, and semiconductor laser light may be raised ina vertical direction by a 45-degree reflection mirror which is formed onthe substrate 3202 of the photodetector by an etching process. While thephotodetector substrate 3202 is fixed on a supporting base plate 3203,this supporting base plate 3203 is adhered to an optical head housing3214 in such a manner that the semiconductor laser chip 3201 is tightlysealed. The optical head housing 3214 is made of either glass or a metalin which a hole is formed at a position through which light passesthrough. The semiconductor laser light penetrates the optical headhousing 3214, and then is reflected from a reflection prism 3205, and isconverted into collimated light by a concave lens 3206 and a convex lens3207. In this case, the concave lens 3206 is employed so as to shorten alength of an optical path. If there is a spare in the optical pathlength, then only the convex lens 3207 may be employed. Next, thecollimated light passes through a liquid crystal phase compensatingelement 3208 and a polarizing diffraction grating 3209, and is convertedfrom linearly polarized light into circularly polarized light by a ¼-λplate 3210. Then, this circularly polarized light is reflected on areflection prism 3211, and then, is condensed onto an optical disk 3216by an objective lens 3212. It should be understood that the size of theliquid crystal phase compensating element 3208 is made larger than adiameter of luminous flux in relation to a dimension of a sealingmember. The reflected light passes through the objective lens 3212 andthe reflection prism 3211, the polarized light becomes linearlypolarized light which is rotated by 90 degrees from a polarizationdirection of luminous flux in the going optical path by the ¼-λ plate3210, and then, this linearly polarized light is diffracted by thepolarizing diffraction grating 3209. The diffracted light passes throughthe liquid crystal compensating element 3208 and then is condensed ontothe photodetector substrate 3202. A signal line from the photodetectorsubstrate 3202 is outputted from a signal terminal 3204 by a bondingwire. In the above-described optical system, since an effective luminousflux diameter in the objective lens 3212 is made smaller than, or equalto 1 mm, the entire optical system can be made compact. Since the entireoptical components are made in an integral form and the integratedoptical components are mounted on an actuator arm 3215, such an adverseinfluence as an optical axis shift of the liquid crystal phasecompensating element 3208 which is caused by the tracking operation canbe eliminated. Furthermore, since the optical head can be made compactand slim, the overall optical disk apparatus can be advantageously madecompact.

[0154] When the effective luminous flux diameter of the objective lensis made smaller than, or equal to 1 mm, how long the interval betweenthe objective lens and the optical disk, namely a working distance canbe secured may constitute a problem. FIG. 33 indicates a calculationresult of the working distance with respective effective diameter undersuch a condition that an NA is 0.85; a thickness of a disk base plate is0.1 mm; a refractive index of the base plate is 1.62; a refractive indexof the objective lens is 1.8; and also, a radius curvature of a firstplane of a single type objective lens is equal to a half of the luminousflux diameter. In this case, based upon Japanese book “Lens DesigningMethod” (written by MATUSI, published by KYORITSU publisher, No. 7,1989), the working distance “WD” may be calculated by the followingexpression 16 under such a lens thickness that a value of sphericalaberration becomes minimum, which is conducted from the aberrationtheory in the analytic manner: $\begin{matrix}{{WD} = {{\left\{ {1 - {\left( {1 - \frac{1}{n}} \right)\frac{t}{R_{1}}}} \right\} f} - \frac{d}{n_{s}}}} & \text{(Expression~~16)}\end{matrix}$

[0155] In this expression, symbol “n” indicates a refractive index of alens, symbol “t” shows a thickness of the lens, symbol “R1” represents afirst plane radius curvature of the lens, symbol “f” shows a focaldistance, symbol “d” denotes a thickness of a disk base plate and also,symbol “ns” indicates a refractive index of the base plate. Such acondition that the first plane radius curvature is equal to ½ of theluminous flux diameter corresponds to a sever condition under which alens can be geometrically established. However, in an actual case, sincethe lens is a non-spherical shape, if a distance is approximated to thisnon-spherical shape, then the lens can be established. As a result, evenwhen NA is equal to 0.85 and the thickness of the base plate is equal to0.1 mm, it can be seen that such a condition may be secured. That is,the effective diameter is 1 mm, and the working distance isapproximately 0.1 mm. This condition is nearly equal to the workingdistance of 0.13 mm as to the two-sheet of lenses having the effectivediameter of 3 mm shown in FIG. 11, namely can be sufficiently realized.

[0156]FIG. 34 shows a laser module, according to an embodiment mode ofthe present invention, in which the semiconductor laser chip 3201 isconstructed on the photodetector substrate 3202 in an integral form. Thedivergent laser light emitted from an edge surface of the semiconductorlaser chip 3201 is vertically raised from the substrate by the 45-degreemirror 3401 which is fabricated by way of the etching process. Since thephotodetector substrate 3202 employs a silicon substrate, if the45-degree mirror 3401 is such a substrate which is cut by shifting thecrystal axial orientation by 9.7 degrees, then the inclination plane of45 degrees may appear by way of the anisotropic etching process.

[0157]FIG. 35 is a diagram for showing a detector pattern and a signalcalculating method in the laser module of FIG. 34. FIG. 36 shows apattern of the polarizing diffraction grating 3209. FIG. 35 also shows apattern of detection light during defocus condition, which is overlappedwith an optical detecting region shown by a hatching manner. All of +first-order diffraction light are connected to each other to form asingle light output so as to output an RF signal by considering such acase that the light reception sensitivity of the photodetector islowered and thus, the S/N ratio of the detector signal is deterioratedwhen a blue-color semiconductor laser is employed as a light source. Asa consequence, an increase of amplifier noise may be suppressed. On theother hand, in order that a spherical aberration signal (SA signal) anda lens shift signal (LS signal) are acquired only from − first-orderdiffraction light by using a small number of splitting lines as beingpermitted as possible, both a focal error signal (AF signal) and an SAsignal are produced by employing an upper half portion of an outer-sidedluminous flux and a lower half portion of an inner-sided luminous flux.A tracking error signal (TR signal) and the lens shift signal (LSsignal) are detected by splitting a lower half portion of theouter-sided luminous flux and an upper half portion of the inner-sidedluminous flux along a radial direction, respectively.

[0158] (Embodiment 8)

[0159]FIG. 37 shows a compact optical disk apparatus, according to anembodiment mode of the present invention, which is arranged by thecompact optical head 3701 of FIG. 32. FIG. 37(a) is a plan view of thecompact optical disk apparatus, and FIG. 37(b) is a side view thereof.The compact optical head 3701 is mounted on an actuator arm 3215, andthe actuator arm 3215 may be moved in a fine mode by the two-dimensionalactuator 3707 along an optical axis direction of an objective lens ofthe optical head, and also, along the radial direction of he opticaldisk 3702. Furthermore, while both the actuator arm 3215 and thetwo-dimensional actuator 3707 are fixed on a swing arm 3703 incommination with a counter balance 3705, the swing arm 3703 drives thecompact optical head 3701 by a swing motor 3704 along the radialdirection of the optical disk 3213. The optical disk 3213 is rotated bya spindle motor 3702. Signal inputs/outputs to the optical head areconnected to a control circuit 3706 by a flexible plastic cable (notshown).

[0160] (Embodiment 9)

[0161]FIG. 38 represents an optical disk apparatus, according to anembodiment mode of the present invention, in such a case that an opticaldisk is either a groove recording system or a land recording system, andthe track pitch of the optical disk is narrower than that of a landgroove type optical disk. Generally speaking, in a recordable opticaldisk, periodic guide grooves are employed along a radial direction ofthis recordable optical disk for a tracking operation. When a trackingdetection by a guide groove is carried out, a push-pull method isemployed. This push-pull method owns such a problem that an optical spoton a photodetector is moved in connection with a tracking operation, andthus, an offset may occur. To mitigate this offset problem, such amethod has been employed in a DVD-RAM and the like. That is, apolarizing diffraction grating is mounted on an objective lens, and aline for splitting luminous flux is not moved with respect to theluminous flex. However, this method is not always operated under perfectcondition, but cannot remove such an offset that an intensity center ismoved within luminous flux in conjunction with a tracking operation.This offset problem does not occur in a land groove type optical disk inwhich an amplitude of a push-pull signal can be increased. However,since an amplitude of a push-pull signal is decreased in either a grooverecording type optical disk or a land recording type optical disk, inwhich a pitch of guide grooves must be made narrow, an offset which isrelatively increased by an intensity distribution shift is notnegligible. As another system capable of reducing the offset of thepush-pull tracking type optical disk, there is a differential push-pullsystem. This differential push-pull system is such a system that since asub-spot is arranged on a disk plane by a shift of ½ track with respectto a main spot, polarities of push-pull signals of both the main spotand the sub-spot obtained on a detector are inverted, and a differentialoutput is obtained from the polarity-inverted push-pull signals, bywhich offsets mixed into both the main spot and the sub-spot in the samephase can be removed. In accordance with this differential push-pullsystem, this system can also cancel such an offset which is caused by anintensity distribution shift which is relatively increased in an opticaldisk having a narrow track pitch. FIG. 38 indicates such an embodimentmode that both spherical aberration and a lens shift are detected in anoptical head using such a differential push-pull system.

[0162] In FIG. 38, light emitted from the semiconductor laser 101 passesthrough the liquid crystal phase compensating element 2901, thediffraction grating 3801, and the polarization beam splitter 1406, andthen, is collimated into parallel light by the collimating lens 102. Inthe diffraction grating 3801, since the sub-spot is formed on the diskplane, diffraction light (not shown) is produced. The collimated beam isfurther converted into circularly polarized light by the ¼-λ plate 1407,and then, this circularly polarized light is condensed onto the opticaldisk 108 by the objective lens 1411 mounted on the two-dimensionalactuator 104. The reflection light is converted by the same ¼-λ plate1407 into linearly polarized light whose polarization direction islocated perpendicular to that of the light when this light is enteredinto this ¼-λ plate 1407. Then, this linearly polarized light isreflected on the polarization beam splitter 1406, and the reflectedlight is received by the photodetector 3802 by applying thereto aastigmatism by the cylindrical lens 111. From an output signal of thephotodetector 3802, a focal shift signal, a tracking signal, a sphericalaberration signal, a lens shift signal, and an RF signal are detectedfrom an AF circuit 113, a TR circuit 114, an SA circuit 115, a Comacircuit 2902, and an RF circuit 116 by way of a calculation,respectively. All of these signals other than the RF signal are fed backto the two-dimensional actuator 104 and the liquid crystal phasecompensating element 2901.

[0163]FIG. 39 shows a grating pattern of the diffractive grating 3801 ofFIG. 38. Linear gratings are arranged along an intersection direction insuch a manner that light of a center portion located in the vicinity ofan optical axis is diffracted along the substantially radial directionof the optical axis, and light of a peripheral portion of the opticalaxis is diffracted along the substantially tangential direction withrespect to an incident luminous flex indicated by a broken line.

[0164]FIG. 40 is a diagram for schematically representing shapes ofoptical spots and arranging positions of the optical spots which areformed on the optical disk 108 by the diffraction grating 3801 of FIG.39. The peripheral light which is diffracted along the substantiallytangential direction constitutes spots 4002 a and 4002 b which areconstituted by a relatively small main lobe, and a peripheral side lobe.At this time, the diffraction grating 3801 is adjusted in such a mannerthat the respective sub-spots are arranged on both sides of the mainspot 4001 and are shifted by a half of the track pitch. Each of theluminous flux which is located in the vicinity of the optical axis, andis diffracted along the substantially radial direction, is separatedfrom each other along the radial direction and forms a large-sized spotby which a structure of a guide groove cannot be resolved.

[0165]FIG. 41 is a schematic diagram for indicating a light receivingplane pattern and detected luminous flux of the photodetector 3802 ofFIG. 38. The diffraction light caused by the guide groove is overlappedwith the luminous flux to be detected to form an interference pattern.In this drawing, in order to easily grasp a polarity of a TR signalcalculation, for the sake of convenience, such a case that thisinterference pattern is unbalanced, while supposing that the main spotformed on the disk plane is slightly off-tracked. Also, a distributionof luminous flux on the detector indicates a position of a least circleof confusion of a condensed spot which is adversely influenced by theastigmatism caused by the cylindrical lens 111. As a consequence, thedistribution of the luminous flux constitutes such a distribution thatan intensity distribution in collimated luminous flux is rotated by 90degrees. An AF signal is calculated by way of the astigmatism method byemploying outputs of four diffraction light 4102 a, 4102 b, 4103 a, and4103 b. A TR signal is calculated by adding a push-pull signal of themain spot 4101 to push-pull signals of the four diffraction light, andby subtracting the added signals from each other in such a manner that alight amount difference is absorbed by a gain coefficient G1. An SAsignal is calculated by that a summation between focal shift signals ofthe outer-sided luminous flux 4102 a, and 4102 b is subtracted from asummation between focal shift signals of the inner-sided luminous flux4103 a and 4103 b. An RF signal is calculated from a summation of themain spots 4101. An LS signal is calculated from a summation betweenpush-pull signals of the inner-sided luminous flux 4103 a and 4103 b. Asshown in this drawing, since a boundary between the inner-sided luminousflux and the outer-sided luminous flux is selected in such a manner thatthe diffraction light caused by the disk guide groove is not overlappedwith the inner-sided luminous flux, only an unbalance in intensitydistributions caused by a lens shift can be detected without any outerdisturbance caused by the guide groove. Also, since the diffractionlight caused by the disk guide groove is not overlapped with theinner-sided luminous flux but also the outer disturbance does not occur,the spot produced by the inner-sided luminous flux need not be arrangedon the same track as that of the outer-sided luminous flux. Also, thespot produced by the inner-sided luminous flux can be readily separatedfrom that of the outer-sided luminous flux and can be easily detected,and further, can be arranged in the radial direction along which a fieldangle from the objective lens is small. While these signals arecalculated, all of the output signals derived from the respective lightreceiving regions need not be solely detected, but since the outputswhich are continuously added/subtracted in the same polarity arepreviously wired on the photodetector substrate, a total number ofoutput signal lines may be suppressed to 10 lines. Also, in thisembodiment mode, the focal shift signals are obtained from thesub-spots. This is because a total number of signal lines used toexecute the external calculation when the RF signal is obtained isreduced as many as possible. In such a case that the outputs derivedfrom the light receiving plane are processed at the stage of the lightcurrents (photocurrents), the noise of the adding amplifier is not mixedinto these outputs. However, when the separately outputted signals areadded/subtracted with each other by employing the adding amplifier, therespective output signals should be previously processed by thecurrent-to-voltage converting operations, so that the noise of theamplifier is mixed into these output signals. In order to obtain thefocal shift signal from the main spot 4101, the light receiving regionof the main spot may be subdivided not by 2, but by 4, so that theamplifier noise becomes twice. In such a case that such a signal S/N canbe secured by which the amplifier noise is located within the allowablerange, it is preferable to acquire the AF signal from the zero-orderlight in view of the precision and stability of the focus controloperations.

[0166] The above-described offset problem of the tracking signal inconnection with the tracking control operation may occur, because theposition of the objective lens is relatively moved with respect to thesemiconductor laser and the photodetector. As a consequence, such anoffset problem does not occur in the case of such an optical head inwhich the semiconductor laser, the photodetector, and the objective lensare driven in the integral form, as described with reference to FIG. 32.

[0167] (Embodiment 10)

[0168]FIG. 42 is a diagram for indicating an optical disk apparatusaccording to another embodiment mode of the present invention, in whichthe diffraction grating 3801 employed in the embodiment mode of FIG. 38is arranged in a detection optical system. Since the diffraction grating4201 is arranged in the detection optical system, only a main spot isformed on the optical disk 108. Light entered to this detection opticalsystem is diffracted by the diffraction grating 4201 having the samepattern as that of FIG. 39, and then, inner-sided luminous flux andouter-sided luminous flux are separated from each other in thediffraction light.

[0169]FIG. 43 indicates a light receiving pattern of the photodetector4202. Since the separation of the luminous flux is carried out in thedetection optical system, an interference pattern of the main spot isidentical to an interference pattern of the sub-spot. As a result, sincethe differential push-pull method is not used, an offset of a trackingsignal is compensated by that the LS signal is multiplied by a constantgain, and then, the multiplied LS signal is subtracted from thepush-pull signal of the main spot 4301.

[0170] (Embodiment 11)

[0171]FIG. 44 represents another photodetector pattern employed in theembodiment mode of FIG. 38 as another embodiment mode. In this case,while the linear grating having no split which has been used in theconventional differential push-pull system is used as the diffractiongrating 3801, two sub-spots are arranged on both sides of a main spot onan optical disk by being shifted by a ½ track. As a result, three spots4401, 4402 a, 4402 b are formed on the photodetector, in whichinterference patterns caused by the guide grooves are inverted. A focalshift signal is obtained by performing the normal focal point detectingcalculation by the astigmatism method with employment of the sub-spots4402 a and 4402 b. A tracking signal is obtained by executing thecalculation of the normal differential push-pull method. A lens shift isobtained by adding two push-pull signals having inverted polarities.However, since the inner-sided luminous flux and the outer-sidedluminous flux are not separated, which have been described in thepresent invention, it is difficult to detect spherical aberration.Therefore, the spherical aberration is detected from a defocuscharacteristic of a push-pull signal.

[0172]FIG. 45 indicates a calculation result of such a defocuscharacteristic of a push-pull signal under such a condition that an NAis 0.85; a wavelength is 0.405 μm; and a track pitch is 0.32 μm. It canbe seen that when a magnitude of spherical aberration is changed, aposition of a peak is shifted. As a result, a signal directlyproportional to the spherical aberration may be acquired by utilizingthis positional shift of the peak as follows: That is, amplitudes ofpush-pull signals are obtained in such a case that a focal point ismoved from a just focusing position along forward/backward directions,and then, a difference is calculated between these amplitudes.

[0173]FIG. 46 shows the spherical aberration signal acquired in theabove-described manner. It can been seen that such a signal which isdirectly proportional to the spherical aberration may be obtained insuch a manner that while a focal shift amount is changed as ±0.25 μm,±0.5 μm, and ±0.75 μm, a difference between amplitudes of push-pullsignals is calculated. However, since the spherical aberration signalcannot be stationarily acquired in this method, this method may beemployed in such a case. That is, when a jump among layers in amulti-layer disk is carried out, spherical aberration is learned beforerecording/reproducing operations are performed, the spherical aberrationis stationarily compensated by using a constant value in the same layer.Also, in this method, signals are similarly obtained even in not onlythe spherical aberration, but also the astigmatism. As a consequence,this method is effective to detect only a change component in thespherical aberration caused by the interlayer jump where the astigmatismis not changed.

[0174] (Embodiment 12)

[0175]FIG. 50 shows a laser module of another embodiment mode, employedin the compact optical head of the embodiment mode shown in FIG. 32. Asthe polarizing diffraction grating 3209, such a polarizing diffractiongrating 5101 indicated in FIG. 51 is employed, and the same astigmatismis applied to both an inner-sided luminous flux and an outer-sidedluminous flux of luminous flux along a 45-degree direction with respectto, radial direction of an optical disk, and further, both theinner-sided luminous flux and the outer-sided luminous flux are enteredinto different light receiving regions on the photodetector. As tooptical spots 5001 a, 5001 b, 5002 a, 5002 b on the photodetector, whenastigmatism having different symbols is applied to + first-orderdiffraction light (5001 a, 5002 a), and − first-order diffraction light(5001 b, 5002 b), and at the same time, when the optical spots on theoptical disk are focused, a least circle of confusion caused by theastigmatism is formed. As a result, interference patterns caused by theguide grooves of the optical disk are rotated by 90 degrees alongopposite directions as to the + first-order light and the − first-orderlight. In the light detecting regions, both the inner-side and theouter-side of the + first-order diffraction light are received by asingle uniform light receiving region, and these inner-sided andouter-sided light outputs are connected on the photodetector so as to beoutputted as an RF signal. Each of inner-sided light and outer-sidedlight as to the − first-order diffraction light is received by a 4-splitlight detecting region, and then, a summation between focal-shiftsignals obtained by the astigmatism method is employed as an AF signal,whereas a subtraction between the focal-shift signals obtained by theastigmatism method is employed as an SA signal. Also, a TR signal may beobtained by that an LS signal acquired by a push-pull signal of theinner-sided luminous flux is multiplied by a constant gain, and theresultant LS signal is subtracted from a push-pull signal of anouter-sided luminous flux. As a consequence, an offset of a lens shiftcaused in the case that the objective lens is not formed with the lasermodule in an integral body can be compensated. It should also be notedthat in the embodiment mode of FIG. 32, these objective lens and lasermodule are formed in the integral form. As a result, the LS signalessentially and continuously becomes zero. Therefore, there is nodifference even when such a calculation is carried out, or not carriedout. When such a calculation is performed, a total number of outputsignal lines including a monitor output lint becomes 10.

[0176] When spherical aberration occurs in the optical spot on the diskplane due to the thickness error of the base plate, if the sphericalaberration is compensated only in the luminous flux which is directed tothe optical disk by employing the previously-explained liquid crystalphase compensating element, then the spherical aberration remains insuch a luminous flux which is returned to the photodetector. FIG. 52indicates a calculation result obtained by calculating this influence inthe focal shift signal. This case supposes such a condition that thefocal point detecting system is the astigmatism method; the NA of theobjective lens is 0.85; the wavelength of the light source is 405 nm;the effective luminous flux diameter of the objective lens is 4 mm; thefocal distance of the condenser lens in the detection system is 20 mm;and the focal point capture range is ±3 μm. When the sphericalaberration caused by the thickness change of the base plate is appliedonly in the detection system within a range of ±1.5 λ (equivalent toapproximately ±11 μm in thickness shift of base plate), it could berecognized that the focal position is shifted by approximately ±0.7 μm.This positional shift corresponds to such a shift amount which is notnegligible. In this case, a relationship of a focal shift signal of afocused position (defocus is 0) with respect to the spherical aberrationamount is represented in FIG. 53. As apparent from this relationship, itcan be seen that a proportional relationship may be established betweenthe focal shift signal and the spherical aberration amount.

[0177] As a result, such an optical disk apparatus as shown in FIG. 54is arranged, and an offset which is directly proportional to a sphericalaberration signal is applied to a focal shift signal. Although anarrangement of an optical system of this optical disk apparatus is madebased upon the optical disk apparatus of FIG. 17 as one example, thespherical aberration caused by the thickness shift of the base plate iscompensated only in the luminous flux of the going optical path, and alleffects can be achieved with respect to the arrangement left in thedetection luminous flux. In this arrangement, first of all, both a focalshift signal of an inner-sided luminous flux and a focal shift signal ofan outer-sided luminous flux are acquired from an output of thephotodetector 1421 by employing a calculation circuit 5401 and anothercalculation circuit 5402. When a difference signal between these focalshift signals is obtained by a differential amplifier 5403, thisdifference signal may constitutes such a spherical aberration signalwhich reflects the spherical aberration occurred on the detector. Astructure for compensating the spherical aberration of the one-wayoptical path from this output is similar to that of FIG. 17. On theother hand, a summation signal between the focal shift signal of theinner-sided luminous flux and the focal shift signal of the outer-sidedluminous flux is calculated by an adder 5404. From this summationsignal, a focal shift signal may be obtained in such a manner that thespherical aberration signal on the detector is multiplied by anamplifier 5405, and then, this multiplied spherical aberration signal issubtracted from this summation signal. Then, the lens actuator 1410 isdriven by this calculated focal shift signal. In the above-describedcircuit arrangement, the AF circuit 113 and the SA circuit 115 areeffectively arranged.

[0178] The above-described calculations are effectively equivalent tosuch a case that in the focal shift signal, a gain distribution ofadding the focal shift signal of the inner-sided luminous flux to thefocal shift signal of the outer-sided luminous flux is changed. Thespherical aberration signal is given as follows:

SA=AF _(in) −AF _(out)  (Expression 17)

[0179] As a result, in the case that the amplification gain of thespherical aberration signal is set to “k” when the spherical aberrationsignal is subtracted from the summation signal, a focal shift signal inthis case is given as follows: $\begin{matrix}\begin{matrix}{{AF} = \quad {{AF}_{in} + {AF}_{out} - {k \cdot {SA}}}} \\{= \quad {{AF}_{in} + {AF}_{out} - {k \cdot \left( {{AF}_{in} - {AF}_{out}} \right)}}} \\{= \quad {{\left( {1 - k} \right){AF}_{in}} + {\left( {1 + k} \right){AF}_{out}}}}\end{matrix} & \text{(Expression~~18)}\end{matrix}$

[0180] In other words, the above-described calculation is equivalent tosuch an operation that the gain of the summation between the focal shiftsignal of the inner-sided luminous flux and the focal shift signal ofthe outer-sided luminous flux is changed. FIG. 55 indicates such acalculation result that an amount obtained by multiplying a sphericalaberration amount by 1.5 is subtracted from a focal shift signal fromFIG. 53. An offset of the focal shift signal is suppressed to asufficiently small value.

[0181] As previously explained, in accordance with the presentinvention, the spherical aberration occurred in the optical diskapparatus can be detected in higher precision and in the easy manner andin low cost. This detected spherical aberration is fed back to thespherical aberration compensating mechanism, so that the quality of thecondensed spot can be maintained in the higher value, and the highdensity recording/reproducing operations of the optical disk can becarried out under stable condition.

INDUSTRIAL APPLICABILITY

[0182] The present invention may be applied to opticalrecording/reproducing operations of information.

1. An optical head comprising: a semiconductor laser; an objective lensfor condensing laser light of said semiconductor laser onto an opticaldisk; a variable focal point applying mechanism for varying a focusposition of the light condensed in an optical system; a sphericalaberration applying mechanism for applying variable spherical aberrationto the light condensed in said optical system; a first optical branchingelement for branching reflection light reflected from said optical diskfrom said optical system; a lens for condensing the branched reflectionlight; and a light receiving element for receiving the light condensedby said lens so as to convert the received light into an electricsignal; wherein: a second branching element is additionally provided inluminous flux of said light, while said second branching elementbranches reflection light to be branched in such a manner that saidsecond branching element further separates the light to be branched intofirst luminous flux located in the vicinity of an optical axis andsecond luminous flux located at a peripheral portion of said opticalaxis, and both said first luminous flux and said second luminous fluxare condensed to said light receiving element.
 2. An optical headwherein: said first optical branching element and said second opticalbranching element are constructed in an integral form.
 3. An opticalhead as claimed in claim 2 wherein: said integrally-formed opticalbranching element is a polarizing diffraction element.
 4. An opticalhead as claimed in claim 1 wherein: said first optical branching elementis a polarizing optical branching element; a ¼-λ plate is providedbetween said first optical branching element and the objective lens;said spherical aberration applying mechanism is a liquid crystalelement; and said liquid crystal element is arranged between saidsemiconductor laser and said first optical branching element.
 5. Anoptical head as claimed in claim 1 wherein: said first optical branchingelement is a non-polarizing optical branching element; and saidspherical aberration applying mechanism is a liquid crystal element;said liquid crystal is arranged between the first optical branchingelement and said objective lens.
 6. An optical head as claimed in anyone of claim 1 to claim 5 wherein: an optical element for producingspherical aberration is not arranged between said first opticalbranching element and said photodetector.
 7. An optical head as claimedin any one of claim 1 to claim 6 wherein: said objective lens and saidspherical aberration applying mechanism are fixed in an integral form.8. An optical head as claimed in claim 7 wherein: an effective luminousflux diameter of said objective lens is smaller than, or equal to 1 mm;said semiconductor laser, said spherical aberration compensatingmechanism, said first and second optical branching elements, saidobjective lens, and said photodetector are fixed in an integral form tobe mounted on the variable focal point mechanism.
 9. An optical head asclaimed in any one of claim 1 to claim 6 wherein: a comatic aberrationmechanism is added to the optical head.
 10. An optical head as claimedin any one of claim 1 to claim 9 wherein: said semiconductor laser isarranged on a base plate where said photodetector is manufactured. 11.An optical head as claimed in any one of claim 1 to claim 10 wherein:said second optical branching element is a diffraction element; isarranged between said semiconductor laser and said objective lens; andis effected in luminous flux directed to said objective lens in such amanner that both + first-order diffraction light and − first-orderdiffraction light of the luminous flux at the peripheral portion arediffracted along a substantially tangential direction, and are arrangedon both sides of zero-order light and essentially shifted by a ½ periodof guide grooves, or pitch columns along a radial direction on anoptical disk, whereas both + first-order diffraction light and −first-order diffraction light of the luminous flux in the vicinity ofthe optical axis are diffracted along the substantially radialdirection.
 12. An optical head as claimed in any one of claim 1 to claim10 wherein: said second optical branching element is a diffractivegrating, separates luminous flux located in the vicinity of said opticalaxis and luminous flux located at the peripheral portion, and at thesame time, owns a pattern which applies astigmatism to the diffractionlight.
 13. An optical disk apparatus comprising: an optical head whichis constituted by: a semiconductor laser; an objective lens forcondensing laser light of said semiconductor laser onto an optical disk;a variable focal point mechanism for varying a focus position of thelight condensed in an optical system; a spherical aberration applyingmechanism for adding variable spherical aberration to the lightcondensed in said optical system; a first optical branching element forbranching reflection light reflected from said optical disk from saidoptical system; a lens for condensing the branched reflection light; anda light receiving element for receiving the light condensed by said lensso as to convert the received light into an electric signal; and inwhich a second branching element is additionally provided in luminousflux of said light, while said second branching element branchesreflection light to be branched in such a manner that said secondbranching element further separates the light to be branched into firstluminous flux located in the vicinity of an optical axis and secondluminous flux located at a peripheral portion of said optical axis, andboth said first luminous flux and said second luminous flux arecondensed to said light receiving element; and a calculation circuit foracquiring both a reproduction signal and a focal shift signal from theelectric signal derived from said light receiving element; wherein: afirst focal shift signal and a second focal shift signal are detected asto each of said first luminous flux and said second luminous flux; saidspherical aberration applying mechanism is controlled based upon asubtraction signal obtained by essentially performing a subtractionbetween said first focal shift signal and said second focal shiftsignal; and said variable focal point mechanism is controlled based upona summation signal obtained between said first focal shift signal andsaid second focal shift signal.
 14. An optical disk apparatus as claimedin claim 13 wherein: in such an arrangement that said sphericalaberration applying mechanism does not apply aberration to projectionlight, but applies the aberration to incident light, and said first andsecond focal shift signals are detected, and then, a sphericalaberration error essentially obtained from the subtraction signalbetween said first and second focal shift signals is fed back to saidspherical aberration adding mechanism, a feedback system is constitutedin such a manner that the spherical aberration on the disk plane becomeszero.
 15. An optical disk apparatus as claimed in claim 14 wherein: saidoptical disk apparatus is provided with a loop in which a drive signalwhich electrically drives said spherical aberration applying mechanismis fed back to a system for amplifying the spherical aberration error.16. An optical disk apparatus as claimed in claim 14, or claim 15wherein: said spherical aberration error is multiplied by a propercoefficient, and said multiplied spherical aberration error is added tothe drive signal of said variable focal point mechanism so as to drivesaid variable focal point mechanism.
 17. An optical disk apparatus asclaimed in claim 13 wherein: said optical disk apparatus is comprisedof: an optical head additionally provided with a tracking controlmechanism for driving said objective lens along a radial direction ofthe optical disk; a calculation circuit for acquiring a reproductionsignal, a focal shift signal, and a tracking error signal from theelectric signal derived from said light receiving element; and a comaticaberration applying mechanism; and further, means for detecting firstand second focal shift signals as to each of said first luminous fluxand said second luminous flux, for controlling said spherical aberrationapplying mechanism based upon a subtraction signal obtained byessentially subtracting both said first and second focal shift signals,for controlling said variable focal point mechanism based upon asummation signal obtained by summing both said first and second focalshift signals, for controlling a tracking control mechanism based uponsaid tracking error signal, for detecting a move amount of the objectivelens during the tracking control operation, and for driving the comaticaberration applying mechanism in response to said move amount.
 18. Anoptical disk apparatus as claimed in claim 17 wherein: said optical headincludes means for splitting said first luminous flux by a diameter ofthe optical disk along a tangential direction and for independentlydetecting the split first luminous flux; and said means for detectingthe move amount of said objective lens detects the move amount bycalculating a difference between two detected light amounts of saidfirst luminous flux which is split to be detected.
 19. An optical diskapparatus as claimed in claim 13 wherein: said optical disk apparatus iscomprised of: an optical head additionally provided with a trackingcontrol mechanism for driving said objective lens along a radialdirection of the optical disk; a calculation circuit for acquiring areproduction signal, a focal shift signal, and a tracking error signalfrom the electric signal derived from said light receiving element; anda comatic aberration applying mechanism; and further, means fordetecting first and second focal shift signals as to each of said firstluminous flux and said second luminous flux, for controlling saidspherical aberration applying mechanism based upon a subtraction signalobtained by essentially subtracting both said first and second focalshift signals, for controlling said variable focal point mechanism basedupon a summation signal obtained by summing both said first and secondfocal shift signals, for controlling a tracking control mechanism basedupon said tracking error signal, for detecting comatic aberration of aspot condensed on the optical disk, and for driving said comaticaberration applying mechanism in response to said detection signal. 20.An optical disk apparatus as claimed in claim 13 wherein: said secondoptical branching element is a diffraction element; is arranged betweensaid semiconductor laser and said objective lens; and is effected inluminous flux directed said objective lens in such a manner that both +first-order diffraction light and − first-order diffraction light of theluminous flux at the peripheral portion are diffracted along asubstantially tangential direction, and are arranged on both sides ofzero-order light and essentially shifted by a ½ period of guide grooves,or pitch columns along a radial direction on an optical disk, whereasboth + first-order diffraction light and − first-order diffraction lightof the luminous flux in the vicinity of the optical axis are diffractedalong the substantially radial direction; said optical head includes atracking control mechanism for driving said objective lens along aradial direction of said optical disk; said optical disk apparatusincludes a calculation circuit for acquiring a reproduction signal, afocal shift signal, and a tracking error signal from the electric signalderived from said light receiving element; said optical disk apparatusdetects a first focal shift signal as to said first luminous fluxlocated in the vicinity of the optical axis and as to said secondluminous flux located at the peripheral portion of the optical axis,which are diffracted by said diffraction grating, so as to control saidspherical aberration applying mechanism based upon a subtraction signalobtained by essentially subtracting both said first and second focalshift signals with each other, and also controls said variable focalpoint mechanism based upon a summation signal obtained by summing bothsaid first and second focal shift signals; and also said optical diskapparatus detects first, second, and third tracking error signals as tosaid first luminous flux, said second luminous flux, and luminous fluxwhich is not diffracted by the diffraction grating so as to control saidtracking control mechanism based upon such a subtraction signal betweenthe third tracking error signal and a summation signal obtained byessentially summing said first and second tracking error signals.
 21. Anoptical disk apparatus comprising: a semiconductor laser; an opticalsystem for condensing laser light of said semiconductor laser onto anoptical disk having a guide groove; an optical branching element forbranching reflected luminous flux to a photodetector; the photodetectorfor detecting the reflected light; a spherical aberration controlmechanism; and a control circuit; wherein; said optical disk apparatusincludes: as means for detecting both a focal shift signal and atracking signal from the output derived from said photodetector to feedback the detected focal shift signal and the detected tracking signal toa lens actuator, means operated in such a manner that after a focuscontrol operation has been commenced with respect to a recording layeron said optical disk, a focal point position is moved from a focusedposition along forward/backward directions, and spherical aberration isdetected from a change in amplitudes of said tracking signal so as todrive said spherical aberration control mechanism.